LIGHT. 



sr distances, through the lessened attrac- 

 tion ? yet these particles, with this amaz- 

 ing 1 motion, will not drive before them, 

 or remove, the least and slightest dust 

 they meet with, and the sun appears to 

 continue of his ancient dimensions, and 

 his attendants move in their ancient or- 

 bits." He therefore conjectures, that all 

 the phenomena of light may be more 

 properly solved, by supposing" all space 

 filled with a subtle elastic fluid, which is 

 not visible when at rest, but which, by 

 its vibrations, affects the fine sense in the 

 eye, as those of the air affect the grosser 

 organs of the ear ; and even that different 

 degrees of the vibration of this medium 

 may cause the appearances of different 

 colours. Franklin's Exper. and Observ. 

 1769, p. 264. 



The celebrated Euler has also main- 

 tained the same hypothesis, in his "Theo- 

 ria Lucis et Colorum." In the summary 

 of his arguments against the common 

 opinion, recited in Acad. Berl. 1752, p. 

 271, besides the objections above-men- 

 tioned, he doubts the possibility, that par- 

 ticles of matter, moving with the amaz- 

 ing velocity of light, should penetrate 

 transparent substances with so much 

 ease. In whatever manner they are 

 transmitted, those bodies must have 

 pores, disposed in right lines, and in all 

 possible directions, to serve as canals for 

 the passage of the rays; but such a struc- 

 ture must take away all solid matter from 

 those bodies, and all coherence among 

 their parts, if they do contain any solid 

 matter. 



Among modern philosophers who 

 have supported this doctrine, Dr. Young- 

 has shown much ability in his experi- 

 mental and theoretical researches, in 

 his memoirs in the "Philosophical Trans- 

 actions," which have been republished 

 in his " Lectures," and in " Nicholson's 

 Journal." 



The expansion or extension of any por- 

 tion of light is inconceivable. Dr. Hook 

 shows, that it is as unlimited as the uni- 

 verse, which he proves from the im- 

 mense distance of many of the fixed stars, 

 which only become visible to the eye by 

 the best telescopes. " Nor," add's he, 

 " are they only the great bodies of the 

 sun or stars that are thus liable to dis- 

 perse their light through the vast expanse 

 of the universe, but the smallest spark of 

 a lucid body must do the same, even the 

 smallest globule struck from u steel by a 

 flint." 



The intensity of different lights, or of 

 the same light in different circumstances, 

 affords a curious subject of speculation. 



M. Bouguer, Trait e de Optique, found, 

 that when one light is from sixty to 

 eighty times less than another, its pre- 

 sence or absence will not be perceived 

 by an ordinary eye ; that the moon's 

 light, -when she is 19 16' high above the 

 horizon, is about one-third of her light, 

 at 66 11' high ; and when one limb just 

 touched the horizon, her light was but 

 the 2,000th part of her light at 66 11' 

 high ; and that hence light is diminished 

 in the proportion of three to one, by tra- 

 versing 7.469 toises of dense air. He 

 found also, that the centre of the sun's 

 disc is considerably more luminous than 

 the edges of it; whereas both the prima- 

 ry and secondary planets are more lumi- 

 nous at their edges than near their cen- 

 tres : that, further, the light of the sun is 

 about 300,000 times greater than that of 

 the moon ; and therefore it is no wonder 

 that philosophers have had so little suc- 

 cess in their attempts to collect the 

 light of the moon with burning glasses ; 

 for, should one of the largest of them 

 even increase the light 1,000 times, it 

 will still leave the light of the moon in 

 the focus of the glass, 300 times less 

 than the intensity of the common light of 

 the sun. 



Dr. Smith, in his optics, vol. i. p. 29, 

 thought he had proved that the light of 

 the full moon would be only the 90,900th 

 part of the full day-light, if no rays were 

 lost at the moon. But Mr. Robins, in his 

 Tracts, vol. ii. p. 225, shows that this is 

 too great by one half. And Mr. Mitchell, 

 by a more easy and accurate mode of 

 Computation, found that the density of 

 the sun's light on the surface of the 

 moon, is but the 45,000th part of the den- 

 sity at the sun ; and that, therefore, as 

 the moon is nearly of the same apparent 

 magnitude as the sun, if she reflected to 

 us all the light received on her surface, 

 it would be only the 45,000th part of our 

 day-light, or that which we receive from 

 the sun. Admitting, therefore,'with M. 

 Bouguer, that the moon's light is only 

 the 300,000th part of the day, or sun's 

 light, Mr. Mitchell concludes that the 

 moon reflects no more than between 

 the 6th and 7th part of what she re- 

 ceives. 



Sir I. Newton long ago observed, that 

 bodies and light act mutually on one an- 

 other; bodies on light, in emitting, re- 

 flecting, refracting, and inflecting it; and 

 light on bodies, by heating them, and put- 

 ting their parts into a vibrating motion, 

 in which heat principally consists. For 

 all fixed bodies, he observes, when ht-ut- 



