OF NEWTON'S OPTICS. 



would be the zenith itself ; for the ray 

 passing from that point to the eye, must 

 enter the atmosphere perpendicularly ; 

 and he observed that in his experiments, 

 whenever the ray met the surface of the 

 water or glass perpendicularly, there was 

 no deflection. 



Thus he perceived that the zenith was 

 a fixed point, with reference to which 

 he should be enabled to ascertain this 

 interesting fact. The test to which he 

 submitted this is a remarkable instance 

 of philosophical acuteness. In his ex- 

 periments he had observed that the 

 more obliquely the ray met the refract- 

 ing surface, the greater was the deflec- 

 tion of the refracted from the incident 

 ray. Hence he supposed that those ob- 

 jects which were more remote from the 

 zenith, and the light of which met the 

 atmosphere more obliquely, were more 

 removed from their proper places. In 

 other words, that the distortion of the 

 firmament by the refraction of the at- 

 mosphere was greater near the horizon 

 than near the zenith. Accordingly, he 

 observed the positions of the same star 

 in different parts of its diurnal path in 

 the heavens, and found that it appeared 

 not to move in a lesser circle parallel to 

 the celestial equator, but that it con- 

 tinually deviated in a slight degree from 

 such a circle ; that this deviation was 

 greater, the more distant the star was 

 from the zenith or highest point, and 

 that the deviation always brought the 

 star nearer to the zenith. 



Such were thephenomenahe observed, 

 and they were precisely what his experi- 

 ments suggested. The last mentioned 

 circumstance, of the deviation being 

 always towards the zenith, showed that 

 the refracted ray was bent towards the 

 perpendicular, which proved that the 

 density of the atmosphere must be greater 

 than that of the fluid, if such there be, 

 which pervades the region beyond it. 



(5.) A long interval elapsed after the 

 age of Ptolemy, before the science of 

 optics made any advance. About the 

 beginning of the twelfth century, some 

 steps were made towards improvement 

 in the theory of vision. In the thirteenth 

 century Roger Bacon devoted consider- 

 able attention to the study of optics ; 

 and although he cannot be said to have 

 extended the bounds of the science by 

 positive discoveries, yet he has so plainly 

 described the effects on vision produced 

 by lenses and their combinations, that 

 we cannot, with any regard to justice, 

 deny him a share in the honour of the 



invention of spectacles, telescopes, and 

 microscopes. In describing the effects 

 of convex lenses of glass, he states 

 that "they are useful to old men, and to 

 those that have weak eyes, for they may 

 see the smallest letters sufficiently mag- 

 nified." Respecting the effects of com- 

 binations of lenses, he says " We shall 

 see the object near at hand, or at a dis- 

 tance, and under any angle we please. 

 And thus from an incredible distance we 

 may read the smallest letters, and may 

 number the smallest particles of dust 

 and sand, by reason of the greatness of 

 the angle under which we see them ; and 

 on the contrary, we may not be able to 

 see the greatest bodies just by us, by 

 reason of the smallness of the angles 

 under which they appear; thus a boy 

 may be as big as a giant, and a man as 

 big as a mountain, forasmuch as we 

 may see the man under as great an angle 

 as the mountain, and as near as we 

 please. Thus, also, the sun, moon, and 

 stars, may be made to descend hither in 

 appearance, and to appear over the heads 

 of our enemies, and many things of 

 a like sort, which would astonish un- 

 skilful persons."* 



It will be perceived that Bacon not only 

 describes the effects of telescopes, but 

 also distinctly alludes to their causes. It 

 is very difficult to conceive that he could 

 have written thus, without having actu- 

 ally constructed the instruments, and 

 witnessed the effects which he describes. 



(6.) In the sixteenth century, Mau- 

 rolycus explained, with great exactness, 

 the structure and functions of the eye, 

 more especially of the crystalline humor. 

 He showed that those defects which are 

 called long-sightedness, and short- 

 sightedness, proceeded from too small or 

 too great a refracting power in the eye, 

 and showed how and why these defects 

 were removed by the use of convex and 

 concave lenses. Maurolycus failed to 

 discover the formation of the picture on the 

 retina, and the functions of that coat, from 

 the difficulty of reconciling an inverted 

 image with our perception of erect objects. 



About this time Baptista Porta," a 

 Neapolitan philosopher, invented the 

 camera obscura. He observed that if a 

 small hole be made in the window-shutter 

 of a darkened chamber, the images of 

 external objects will appear depicted in 

 their proper colours on the opposite wall. 

 He then tried the effect of a convex lens 

 fixed in the aperture, and found that the 



* See the Edinburgh Encyclopaedia OPTICS. 

 B 2 



