William Herschel

Sir Frederick William Herschel KH FRS (German: Friedrich Wilhelm Herschel) (15 November 1738 – 25 August 1822) was a German-born British astronomer, technical expert, telescope maker, organist and composer who became famous for discovering Uranus. He also discovered infrared radiation and made many other contributions to astronomy.

His sister Caroline Herschel and son, Sir John Herschel, were also notable astronomers.

• I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet.
  • His discovery of Uranus. Scientific Papers, vol. 1, page 30 "Account of a Comet".

• Hier ist wahrhaftig ein Loch im Himmel !
  • Here is truly a hole in Heaven.
    • as remembered by his sister Caroline, after a long period of scrutinizing a starless spot, probably in the constellation Scorpius. As quoted by Michael J. Crowe (1994). Modern theories of the universe: from Herschel to Hubble. Courier Dover Publications. p. 207. ISBN 0486278808. 

arranged for the Purpose of a critical Examination the Result of which appears to throw some new Light upon the Organization of the celestial Bodies,
Philosophical Transactions of the Royal Society of London, Vol.101

• A knowledge of the construction of the heavens has always been the ultimate object of my observations...

• I must freely confess that by continuing my sweeps of the heavens my opinion of the arrangement of the stars and their magnitudes, and of some other particulars, has undergone a gradual change...

• An equal scattering of the stars may be admitted in certain calculations; but when we examine the milky way, or the closely compressed clusters of stars... this supposed equality of scattering must be given up.

• 'We may... have surmised nebulae to be no other than clusters of stars disguised by their very great distance, but a longer experience and better acquaintance with the nature of nebulae, will not allow a general admission of such a principle, although undoubtedly a cluster of stars may assume a nebulous appearance when it is too remote for us to discern the stars of which it is composed.

• An object may not only contain stars, but also nebulosity not composed of them.

• It will be necessary to explain the spirit of the method of arranging the observed astronomical objects under consideration in such a manner, that one shall assist us to understand the nature and construction of the other. This end I propose to obtain by assorting them into as many classes as will be required to produce the most gradual affinity... and it will be found that those contained in one article, are so closely allied to those in the next, that there is perhaps not so much difference between them... as there would be in an annual description of the human figure were it given from the birth of a child till he comes to be a man in his prime.

• A nebulous matter, diffused in such exuberance throughout the regions of space, must surely draw our attention to the purpose for which it probably may exist; and it must be the business of a critical inquirer to attend to all the appearances under which it will be exposed to his view...

• A proportional condensation of the nebulous matter in the brighter places will sufficiently account for their different degree of shining.

• Instead of inquiring after the nature of the cause of the condensation of nebulous matter, it would indeed be sufficient for the present purpose to call it merely a condensing principle; but since we are already acquainted with the centripetal force of attraction which gives a globular figure to planets, keeps them from flying out of their orbits in tangents, and makes one star revolve around another, why should we not look up to the universal gravitation of matter as the cause of every condensation, accumulation, compression, and concentration of the nebulous matter?

• The number of compound nebulæ... being so considerable, it will follow, that if they owe their origin to the breaking up of some former extensive nebulosities of the same nature with those which have been shewn to exist at present, we might expect that the number of separate nebulæ should far exceed the former, and that moreover these scattered nebulas should be found not only in great abundance, but also in proximity or continuity with each other... Now this is exactly what by observation, we find to be the state of the heavens.

• We may conceive that, perhaps in progress of time these nebulæ which are already in such a state of compression, may be still farther condensed so as actually to become stars.

• We can hardly suppose a possibility of the production of a globular form without a consequent revolution of the nebulous matter, which in the end may settle in a regular rotation about some fixed axis.

• I compared also the present appearance of this nebula with the delineation which Huyghens has given of it in his Systema Saturnium... The changes that are thus proved to have already happened, prepare us for those that may be expected hereafter to take place, by the gradual condensation of the nebulous matter; for had we no where an instance of any alteration in the appearance of nebula, they might be looked upon as permanent celestial bodies, and the successive changes, to which by the action of an attracting principle they have been conceived to be subject, might be rejected as being unsupported by observation.

• The starlike appearance of the following six nebulæ is so considerable that the best description... was to compare them to stars with certain deficiencies.

as quoted by Edward Singleton Holden

• I have made it a rule never to employ a larger telescope when a smaller will answer the purpose.
  • Ch.4 "Life and Works".

• When I resided at Bath I had long been acquainted with the theory of optics and mechanics, and wanted only that experience so necessary in the practical part of these sciences. This I acquired by degrees at that place where in my leisure hours, by way of amusement, I made several two-foot, five-foot, seven-foot, ten-foot and twenty-foot Newtonian telescopes, beside others, of the Gregorian form, of eight, twelve, and eighteen inches, and two, three, five, and ten feet focal length. In this way I made not less than two hundred seven-foot, one hundred and fifty ten-foot, and about eighty twenty-foot mirrors, not to mention the Gregorian telescopes.*
  • Ch.4 "Life and Works" Footnote: At least one of these telescopes had the principal mirror made of glass instead of metal. Philosophical Transactions of the Royal Society of London (1803).

• The number of stands I invented for these telescopes it would not be easy to assign. ...In 1781 I began to construct a thirty foot aërial reflector, and having made a stand for it, I cast the mirror thirty-six inches in diameter. This was cracked in cooling. I cast it a second time, and the furnace I had built in my house broke.
  • Ch.4 "Life and Works".

• In the year 1783 I finished a very good twenty-foot reflector with a large aperture, and mounted it upon the plan of my present telescope. After two years' observation with it, the great advantage of such apertures appeared so clearly to me that I recurred to my former intention of increasing them still further; and being now sufficiently provided with experience in the work which I wished to undertake, the President of the Royal Society, who is always ready to promote useful undertakings, had the goodness to lay my design before the king. His Majesty was graciously pleased to approve of it, and with his usual liberality to support it with his royal bounty.
  • Ch.4 "Life and Works".

• In consequence of this arrangement I began to construct the forty-foot telescope about the latter end of 1785. The woodwork of the stand and machines for giving the required motions to the instrument were immediately put in hand. In the whole of the apparatus none but common workmen were employed, for I made drawings of every part of it, by which it was easy to execute the work, as I constantly inspected and directed every person's labor; though sometimes there were not less than forty different workmen employed at the same time. While the stand of the telescope was preparing, I also began the construction of the great mirror, of which I inspected the casting, grinding, and polishing, and the work was in this manner carried on with no other interruption than that occasioned by the removal of all the apparatus and materials from where I then lived, to my present situation at Slough.
  • Ch.4 "Life and Works"

• Here [in Slough], soon after my arrival, I began to lay the foundation upon which by degrees the whole structure was raised as it now stands, and the speculum being highly polished and put into the tube, I had the first view through it on February 19, 1787. ...the first speculum, by a mismanagement of the person who cast it, came out thinner on the centre of the back than was intended, and on account of its weakness would not permit a good figure to be given to it. ...A second mirror was cast January 26, 1788, but it cracked in cooling. February 16 we recast it, and it proved to be of a proper degree of strength. October 24 it was brought to a pretty good figure and polish, and I observed the planet Saturn with it. But not being satisfied, I continued to work upon it till August 27, 1789, when it was tried upon the fixed stars, and I found it to give a pretty sharp image. Large stars were a little affected with scattered light, owing to many remaining scratches on the mirror. August the 28th, 1789, having brought the telescope to the parallel of Saturn, I discovered a sixth satellite of that planet, and also saw the spots upon Saturn better than I had ever seen them before, so that I may date the finishing of the forty-foot telescope from that time.
  • Ch.4 "Life and Works".

• I should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun. ...The spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things. ...The sun itself has been called a globe of fire, though, perhaps, metaphorically. ...It is time now to profit by the observations we are in possession of. I have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena.
  • Ch.4 "Life and Works" quote from his paper "Nature and Construction of the Sun and Fixed Stars" (1795).

• According to my theory, a dark spot in the sun is a place in its atmosphere which happens to be free from luminous decompositions [above it].
  • Ch.4 "Life and Works" quote from his paper "Nature and Construction of the Sun and Fixed Stars" (1795).

• That the emission of light must waste the sun, is not a difficulty that can be opposed to our hypothesis. Many of the operations of Nature are carried on in her great laboratory which we cannot comprehend. Perhaps the many telescopic comets may restore to the sun what is lost by the emission of light.
  • Ch.4 "Life and Works" quote from his paper "Nature and Construction of the Sun and Fixed Stars" (1795).

• These [binary stars] may serve another very important end. ...Several stars of the first magnitude have been observed or suspected to have a proper motion; hence we may surmise that our sun, with all its planets and comets, may also have a motion towards some particular point of the heavens. ...If this surmise should have any foundation, it will show itself in a series of some years in a kind of systematical parallax, or change due to the motion of the whole solar system.
  • Ch.4 "Life and Works" quote from "Researches on the Motion of the Sun and of the Solar System in Space" (1782).

• In future... we shall look upon those regions into which we may now penetrate by means of such large telescopes, as a naturalist regards a rich extent of ground or chain of mountains containing strata variously inclined and directed, as well as consisting of very different materials. The surface of a globe or map therefore will but ill delineate the interior parts of the heavens.
  • Ch.4 "Life and Works" from a memoir (1784).

• It is very probable that the great stratum called the Milky Way is that in which the sun is placed, though perhaps not in the very centre of its thickness. ...We gather this from the appearance of the Galaxy, which seems to encompass the whole heavens, as it certainly must do if the sun is within it.
  • Ch.4 "Life and Works".

• It is evident that we cannot mean to affirm that the stars of the fifth, sixth, and seventh magnitudes are really smaller than those of the first, second, or third, and that we must ascribe the cause of the difference in the apparent magnitudes of the stars to a difference in their relative distances from us. On account of the great number of stars in each class, we must also allow that the stars of each succeeding magnitude, beginning with the first, are, one with another, further from us than those of the magnitude immediately preceding.
  • Ch.4 "Life and Works" from a memoir, published (1817).

• A standard of reference for the arrangement of the stars may be had by comparing their distribution to a certain properly modified equality of scattering. The equality which I propose does not require that the stars should be at equal distances from each other, nor is it necessary that all those of the same nominal magnitude should be equally distant from us.
  • Ch.4 "Life and Works" from a memoir, published (1817).

• In this case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light; that is to say, of rays coming from the sun, that have such a momentum as to be unfit for vision. And admitting, as is highly probable, that the organs of sight are only adapted to receive impressions from particles of a certain momentum, it explains why the maximum of illumination should be in the middle of the refrangible rays; as those which have greater or less momenta are likely to become equally unfit for the impression of sight.
  • Ch.4 "Life and Works" on his discovery of the infrared light.

• To conclude, if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired whether light be essentially different from radiant heat? In answer to which I would suggest that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one. ...If this be a true account of the solar heat, for the support of which I appeal to my experiments, it remains only for us to admit that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted under the appearance of light and colors, and that the rest, being stopped in the coats and humors of the eye, act on them, as they are known to do on all the other parts of our body, by occasioning a sensation of heat.
  • Ch.4 "Life and Works" on his discovery of the infrared light.

• This consideration must alter the form of our proposed inquiry; for the question being thus at least partly decided, since it is ascertained that we have rays of heat which give no light, it can only become a subject of inquiry whether some of these heat-making rays may not have a power of rendering objects visible, superadded to their now already established power of heating bodies. This being the case, it is evident that the onus probandi [burden of proof] ought to lie with those who are willing to establish such an hypothesis, for it does not appear that Nature is in the habit of using one and the same mechanism with any two of our senses. Witness the vibration of air that makes sound, the effluvia that occasion smells, the particles that produce taste, the resistance or repulsive powers that affect the touch—all these are evidently suited to their respective organs of sense.
  • Ch.4 "Life and Works" on his discovery of the infrared.

• Nebulæ can be selected so that an insensible gradation shall take place from a coarse cluster like the Pleiades down to a milky nebulosity like that in Orion, every intermediate step being represented. This tends to confirm the hypothesis that all are composed of stars more or less remote.
  • Ch.4 "Life and Works"

• The naked eye has its limit of vision in the stars of the sixth magnitude. The light of fainter stars than these does not affect the retina enough for them to be seen. A very small telescope penetrates to smaller, and, in general, without doubt, to more distant stars. A more powerful one penetrates deeper into space, and as its power is increased, so the boundaries of the visible universe are widened, and the number of stars increased to millions and millions. Whoever has followed the history of the series of Herschel's telescopes will have observed this. But Herschel was not content with the bare fact, but strove ever to know how far a telescope of a certain construction and size could penetrate, compared with the naked and unassisted eye. These investigations were never for the discovery of new facts concerning the working of his instruments; it was for the knowledge of the distribution of the fixed stars in space itself that he strove. Herschel's instruments were designed to aid vision to the last extent. They were only secondarily for the taking of measures. His efforts were not for a knowledge of the motions, but of the constitution and construction of the heavenly bodies.
  • Friedrich Bessel, Königsberger All genuine Zeitung, I (1843) No.37 as quoted by Edward Singleton Holden, Sir William Herschel: His Life and Works (1881).

• Like Bradley, Herschel made an unexpected discovery in searching for parallax. He selected for observation stars close to each other in the sky, following the guidance of Galileo. Although Herschel failed to find parallax, he did find that in some cases the stars he observed appeared to be in relative motion around their common center of gravity. They were clearly "binary stars," bound by gravity to orbit one another. It is now realized that almost half of all stars can be found in binary systems.
  • David H. Clark & Matthew D. H. Clark, Measuring the Cosmos: How Scientist Discovered the Dimensions of the Universe (2004).

• Being self-taught, he had not known that astronomers were expected to focus on the solar system. Instead, he explored the construction of the universe, and it was on later generations that the questions he asked and the methods he devised to answer them were to have profound influence.
  • Michael Hoskin, ‘Herschel, William (1738–1822)’, Oxford Dictionary of National Biography, Oxford University Press, 2004.

• William as a natural historian of the heavens, a collector of astronomical specimens, was like a modern supertanker; once under way, it was almost impossible for him to stop. In 1802, when the campaign with the 20ft finally came to an end, the hundred or so nebulae of Messier had been augmented by no fewer than two-and-a-half thousand.
  • Michael Hoskin, "Unfinished Business: William Herschel's Sweeps for Nebulae" History of Science (2005) Vol. 43, part 3.

• William's catalogues [of nebulae] are... of the greatest significance in the story of the astronomy of the large-scale universe.
  • Michael Hoskin, "Unfinished Business: William Herschel's Sweeps for Nebulae" History of Science (2005) Vol. 43, part 3.

• In 1800 he had published his momentous discovery of infra-red rays; and in 1803 and 1804 his re-examination of double stars would reveal examples where the two components had orbited each other, visual proof that attractive forces...operated outside the solar system. ...between 1811 and 1818 he published four great synthetic papers on the construction of the heavens, in which he expounded the life-story of nebulae and clusters as they developed over time under the influence of gravity. ...Soon, development over time—in contrast to the unchanging clockwork universe of Newton and Leibniz—would become and remain part of astronomical thinking.
  • Michael Hoskin, "Unfinished Business: William Herschel's Sweeps for Nebulae" History of Science (2005) Vol. 43, part 3.

• It's no exaggeration to say that modern astronomy was invented, more or less single-handedly, by William Herschel in the last decades of the eighteenth century.
  • Michael D. Lemonick, The Georgian Star: How William and Caroline Herschel Revolutionized Our Understanding of the Cosmos (2009).

• William's chief reference... was A Complete System of Opticks, a two-volume work published in 1738 by Robert Smith, the same author whose [mathematics] book on harmonics [Harmonics, or The Philosophy of Musical Sounds] had captivated him more than a decade earlier. Smith explained the theory of optics but also gave step-by-step instructions on how to apply it to the construction and use of astronomical telescopes. ...word reached him that a Quaker gentleman who lived nearby was giving up his hobby, which just happened to be grinding telescope mirrors. William bought up his tools and some partially finished mirrors. The man gave him some lessons and then, armed with Smith's instructions, he went to work.
  • Michael D. Lemonick, The Georgian Star: How William and Caroline Herschel Revolutionized Our Understanding of the Cosmos (2009).

• William Herschel was the first man to give a reasonably correct picture of the shape of our star-system or galaxy; he was the best telescope-maker of his time, and possibly the greatest observer who ever lived.
  • Patrick Moore, astronomer, as quoted on the official website of the Royal Borough of Windsor and Maidenhead.

• "Planetarium" was written after a visit to a real planetarium, where I read an account of the work of Caroline Herschel, the astronomer, who worked with her brother William, but whose name remained obscure, as his did not.
  • Adrienne Rich Arts of the Possible (2001)

• In consequence of the harassing and fatiguing life he had led during the winter months, he used to retire to bed with a bason of milk or glass of water, and Smith's 'Harmonics and Optics,' Ferguson's 'Astronomy,' &c., and so went to sleep buried under his favourite authors; and his first thoughts on rising were how to obtain instruments for viewing those objects himself of which he had been reading.
  • Ch.2 "Life in Bath" pp.34–5, Ref. James Ferguson, Astronomy: explained upon Sir Isaac Newton's Principles (1756) Vol.1, Vol.2

• It soon appeared that my brother was not contented with knowing what former observers had seen, for he began to contrive a telescope eighteen or twenty feet long (I believe after Huyghens description)...
  • Ch.2 "Life in Bath".

• I was much hindered in my musical practice by my help being continually wanted in the execution of the various contrivances, and I had to amuse myself with making the tube of pasteboard for the glasses which were to arrive from London, for at that time no optician had settled at Bath. But when all was finished, no one besides my brother could get a glimpse of Jupiter or Saturn, for the great length of the tube would not allow it to be kept in a straight line. This difficulty, however, was soon removed by substituting tin tubes.
  • Ch.2 "Life in Bath".

• My brother wrote to inquire the price of a reflecting mirror for (I believe) a five or six foot telescope. The answer was, there were none of so large a size, but a person offered to make one at a price much above what my brother thought proper to give... About this time he bought of a Quaker resident at Bath, who had formerly made attempts at polishing mirrors, all his rubbish of patterns, tools, hones, polishers, unfinished mirrors, &c., but all for small Gregorians, and none above two or three inches diameter.
  • Ch.2 "Life in Bath".

• To my sorrow I saw almost every room turned into a workshop. A cabinetmaker making a tube and stands of all descriptions in a handsomely furnished drawing-room. Alex [a brother] putting up a huge turning machine (which he had brought in the autumn from Bristol where he used to spend the summer) in a bedroom, for turning patterns, grinding glasses, and turning eye-pieces &c. At the same time music durst not lie entirely dormant during the summer, and my brother had frequent rehearsals at home...
  • Ch.2 "Life in Bath"

• According to Herschel, the sun consisted of three essentially different parts. First, there was a solid nucleus, non-luminous, cool, and even capable of being inhabited. Second, above this was an atmosphere proper; and, lastly, outside of this was a layer in which floated the clouds, or bodies which gave to the solar surface its intense brilliancy.
  • Ch.4 "Life and Works".

• In 1783 he published his paper On the Proper Motion of the Solar System which contained the proofs of his surmises of a year before. ...His second paper on the Direction and Velocity of the solar system (1805) is the best example that can possibly be given of his marvellous skill in reaching the heart of a matter, and it may be the one in which his philosophical powers appear in their highest exercise. For sustained reflection and high philosophic thought it is to be ranked with the researches of Newton in the Principia.
  • Ch.4 "Life and Works".

• Herschel's method of study was founded on a mode of observation which he called star-gauging. It consisted in pointing a powerful telescope toward various parts of the heavens, and ascertaining by actual count how thick the stars were in each region. His twenty-foot reflector was provided with such an eye-piece that, in looking into it, he saw a portion of the heavens about 15' in diameter. A circle of this size on the celestial sphere has about one quarter the apparent surface of the sun, or of the full moon. On pointing the telescope in any direction, a greater or less number of stars were visible. These were counted, and the direction in which the telescope pointed was noted. Gauges of this kind were made in all parts of the sky, and the results were tabulated in the order of right ascension.
  • Ch.4 "Life and Works".

• Herschel's argument was... Since with such a telescope one can see a star ten times as far off as is possible to the naked eye, this telescope has the power of penetrating into space ten times farther than the eye alone. But this number ten, also, expresses the ratio of the diameter of the objective to that of the pupil of the eye, consequently the general law is that the space-penetrating power of a telescope is found by dividing the diameter of the mirror in inches by two-fifths. The diameter of the pupil of the eye (two fifths of an inch) Herschel determined by many measures.
  • Ch.4 "Life and Works".

• Having tried many varieties of shade-glasses between the eye-piece of his telescope and the eye, in order to reduce the inordinate degree of heat and light transmitted by the instrument when directed towards the sun, he observed that certain combinations of colored glasses permitted very little light to pass, but transmitted so much heat that they could not be used; while, on the other hand, different combinations and differently colored glasses would stop nearly all the heat, but allow an inconveniently great amount of light to pass. At the same time he noticed, in the various experiments, that the images of the sun were of different colors. This suggested the question as to whether there was not a different heating power proper to each color of the spectrum. On comparing the readings of sensitive thermometers exposed in different portions of an intense solar spectrum, he found that, beginning with the violet end, he came to the maximum of light long before that of heat, which lay at the other extremity, that is, near the red. By several experiments it appeared that the maximum of illumination, i.e., the yellow, had little more than half the heat of the full red rays; and from other experiments he concluded that even the full red fell short of the maximum of heat, which, perhaps, lay even a little beyond the limits of the visible spectrum.
  • Ch.4 "Life and Works".

• A third and last paper in this department of physics [the infrared and light spectrum]... was published in volume ninety of the Philosophical Transactions and gave the results of two hundred and nineteen quantitative experiments. ...Herschel made a careful determination of the quantitative distribution of light and of heat in the prismatic spectrum, and discovered the surprising fact that not only where the light was at a maximum the heat was very inconsiderable, but that where there was a maximum exhibition of heat, there was not a trace of light. ...Herschel ...finally concluded that light and radiant heat were of essentially different natures... for a long time the question was looked upon as closed, and not until thirty-five years later was there any dissent. Then the Italian physicist, Melloni, with instrumental means a thousand times more delicate than that of Herschel, and with a far larger store of cognate phenomena, collected during the generation which had elapsed... discovered the true law.
  • Ch.4 "Life and Works" Ref. "Experiments on the solar and on the terrestrial Rays that occasion Heat..." Philosophical Transactions of the Royal Society Vol.90

• Groups which remain nebulous in a seven-foot telescope, become stellar in a ten-foot. The nebulosity of the ten-foot can be resolved into stars by the twenty-foot, and so on. The nebulæ which remained still unresolved, it was reasonable to conclude, would yield to higher power, and generally a nebula was but a group of stars removed to a great distance. An increase of telescopic power was alone necessary to demonstrate this. So, at first, Herschel believed that his twenty-foot telescope was of power sufficient to fathom the Milky Way, that is, to see through it and beyond it, and to reduce all its nebulosities to true groups of stars. In 1791 he published a memoir on Nebulous Stars, in which his views were completely changed. He had found a nebulous star... to which his reasons would not apply. In the centre of it was a bright star; around the star was a halo gradually diminishing in brightness from the star outward, and perfectly circular. It was clear the two parts, star and nebula, were connected and thus at the same distance from us. ...The hypothesis of an elastic shining fluid existing in space, sometimes in connection with stars, sometimes distinct from them, was adopted and never abandoned. ...in late years we have seen the reverse of the process imagined by Herschel. A star has actually, under our eyes, become a planetary nebula, and the cycle of which he gave the first terms is complete.
  • Ch.4 "Life and Works"

• The Georgian Star: how William and Caroline Herschel invented modern astronomy from the Royal Society Channel @YouTube
• Sir William Herschel: his Life and Works (1881) Edward Singleton Holden @archive.org
• Sir William Herschel: His Life and Works (1880) Edward Singleton Holden @GoogleBooks
• A Synopsis of the Scientific Writings of Sir William Herschel Edward Singleton Holden, William Singleton Holden & Charles Sheldon Hastings
• William Herschel: his music, from the BBC
• Infrared Thermometer Applications