798 



SOLAR RAYS SOTHEBY. 



SOLAR RAYS. We take the opportunity af- 

 forded us under this head of making a few remarks 

 upon the concentration of the sun's rays by burn- 

 ing glasses, with the view to explain more clearly 

 than our previous articles have done, the rationale 

 of their fusing power. If the reader refers to 

 Burning Lens, and Speculum, he will find nothing 

 to determine the power beyond the long recog- 

 nized law that the density of the foci in any two 

 lenses is directly as the areas of the glasses, and in- 

 verselyta the squares of the focal length. Experience 

 shows, however, that this law is insufficient to de- 

 termine the fusing power ; for if we take lenses 

 perfectly proportional in area and focal length, we 

 get focal images of equal intensity, which notwith- 

 standing act very unequally on the substances ex- 

 posed to them. In Lardner's Cyclopedia, vol. 19, 

 p. 131, we find the following remarks by Brewster, 

 illustrative of what we have just mentioned, and 

 serving at the same time to show that the different 

 powers of images intensively the same, but exten- 

 sively different, are at the present day regarded as 

 inexplicable phenomena : 



" M. Buffon found that a convex lens, of long 

 focal length, was preferable to one of a short focal 

 length for fusing metals by the concentration of 

 the sun's rays. A lens, for example, 32 inches in 

 diameter, and 6 inches in focal length, with the 

 diameter of its focus 8 lines, melted copper in less 

 than a minute ; while a small lens, 32 lines in dia- 

 meter, with a focal length of 6 lines, and its focus 

 of a line, was scarcely capable of heating copper." 



This passage does indeed relate an inexplicable 

 phenomenon unless we exchange the italics for 6 

 feet and 6 inches; for never had lenses such mar- 

 vellous foci before. Our correction agrees with 

 the proportion between the focal length and the 

 diameter of the focal image, which is 108 : 1. We 

 are surprised to hear Sir D. Brewster speak of 

 focal length as the measure of burning power : in 

 the present case, it would have been more philoso- 

 phical to have said, that M. Buffon found a convex 

 lens of large surface much preferable to one of 

 small surface, when the surfaces, foci, and focal 

 images of the two were strictly proportional. 



Now according to the law before named, the 

 burning power of these two lenses should be the 

 same. Let P equal the power of the larger lens, 

 and ;j that of the smaller one 



Then 32 s : 32 -^ 12 2 : : 6* : 72 



And P : p : 



That is P : p 



32 X 6* : 32 -j- 12* X 72* 

 : : 36864 : 36864 i. e. as 1 : 1. 



Buffon, however, found that the fusing power 

 was incomparably greater in the larger than in the 

 smaller lens. How then are we to account for 

 this difference of action? We naturally look to 

 the relative magnitudes of the focal images; and a 

 little consideration will assure us, that though 

 their intensity is the same, space for space, the 

 largest image must produce considerably more 

 effect than the other; because the heat at the centre 

 of the large image cannot be dissipated, or con- 

 ducted away, nearly so rapidly as the heat at the 

 centre of the small one. If we are not mistaken, 

 the fusing power of the focal images formed by the 

 two lenses quoted by Brewster, is directly as the 

 squares of the radii of the images. Hence we 

 say : 



P : p : : 4 : f 



That-is, P : p : : 16 : ^ or 144 : 1. 



Mr Pritchard, one of the first authorities of the 



day in all optical matters, remarks, in a recent 

 letter to us, " With respect to burning lenses, 

 it appears to me that no experiments have been 

 made with sufficient accuracy to deduce a formula 

 from them." He then alludes to the observations 

 of Herschel and Goring in the " Micrograpbia," 

 regarding focal images: we do not, however, find 

 in them any thing that goes beyond the established 

 law stated at the commencement of this article. 

 Under present circumstances we offer our formula 

 for determining the intensity, as an incentive to 

 accurate experiment by which alone it can be pro- 

 perly tested. 



SOTHEBY, WILLIAM, a gentleman of liberal 

 fortune and education, who distinguished himself 

 by his poetical translations, was born in London on 

 the 9th of November, 1757. By the death of his 

 father, when only seven years old, he was left un- 

 der the guardianship of the honourable Charles 

 Yorke, afterwards lord Chancellor, and of his ma- 

 ternal uncle, Hans Sloane, Esq. By them he was 

 placed at Harrow, where he remained till the age 

 of seventeen, when that active disposition which ac- 

 companied him through life induced him to enter the 

 army, instead of completing his education at either 

 of the universities. He purchased a commission in 

 the tenth dragoons, from which he immediately 

 obtained leave of absence, and passed several 

 months at the military academy at Angers, for the 

 purpose of more fully studying the principles of 

 his profession. On quitting Angers, he spent the 

 following winter and spring in the brilliant societies 

 of Vienna and Berlin, and, returning through the 

 South of France to England, rejoined his regiment 

 towards the close of 1777. Upon his marriage in 

 July, 1780, he quitted the army, and purchased 

 Bevis Mount, near Southampton, where he con- 

 tinued to reside for the next ten years. 



He now principally devoted his time to the dili- 

 gent study of the classics, to the translation of 

 many of the minor Greek and Latin poets, and 

 some original compositions, which his maturer 

 taste did not deem worthy of publication. In 

 1788, he made a pedestrian tour through Wales, 

 which gave rise to some odes and sonnets, with a 

 poetical description of that romantic country, pub- 

 lished in 1789, under the title of " A Tour through 

 North and South Wales." In 1791, he removed to 

 London, and from this time made the metropolis his 

 principal place of residence. He was now elected 

 a fellow of the royal and antiquarian societies, and 

 became a member of the Dilettanti, and several 

 other literary and scientific meetings. At his own 

 house he was also in the constant habit of receiv- 

 ing persons of talent of all parties, both in politics 

 and in literature, where the warmth of his manner 

 and cheerful tone of his mind, threw a peculiar charm 

 over the society. In 1798, he published a transla- 

 tion from the Oberon of Wieland, which became 

 immediately popular ; and in 1799, a poem on the 

 battle of the Nile. 



In 1800 his well-known translation of the 

 Georgics of Virgil appeared. In 1801, his love 

 of the fine arts prompted him to address his friend 

 Sir George Beaumont, in " A Poetical Epistle on 

 the Encouragement of the British School of Paint- 

 ing." In 1802 he first published the tragedy of 

 " Orestes," on the model of the ancient Greek 

 drama, accompanied by a mask entitled "Huon de 

 Bordeaux," founded on the poem of Oberon, and 

 interspersed with many elegant fairy songs adapted 

 to the music of Viotti. In 1807, he published 



