THE PROGRESS OF 



lion, is reflected br.ck again. The particles, on 

 the other hand, which are in a suite that disposes 

 them to be transmitted, overcome the repulsive 

 force, and entering the transparent hody, arc 

 attracted and refracted as already explained. If 

 these rays reach the second surface of the tnm*,- 

 parent body with a certain obliquity, the attrac- 

 tion will not suffer the rays to emerge into the 

 rarer medium, hut will f.>rce them to return bark 

 into the transparent body. Thus the reflection 

 of light, at the second surface of a transparent 

 body, is prwluced not by the repulsion of the 

 medium into \\liich it is about to enter, but by 

 the attraction of that which it was going to 

 leave. 



In consequence of t'.iese discoveries, Newton 

 perceired a defect in the refracting telescope, 

 proceeding from the unequal refrangibility of the 

 different rays. This defect he was of opinion it 

 was impossible to remedy, because he thought 

 that the quantity of the refraction and of the dis- 

 persion in different substances bore always the 

 same ratio to each other. Had this been so, the 

 appearance of prismatic colours in a refracting 

 telescope could never have been cured without 

 destroying the refraction altogether. But this 

 was one of the few points about which Newton 

 was mistaken. The mistake was afterwards dis- 

 covered by Dollond, who found that, by means 

 of crown and flint glass, the dispersion might be 

 counteracted without injuring the refraction. 

 This great discovery enabled him prodigiously 

 to improve the refracting telescope ; and they are 

 now reckoned so much preferable, that they are 

 superseding the reflecting telescope altogether. 



Newton turned his attention also to the inflec- 

 tion of light. He found that the ray, in passing 

 the knife edge, had been both attracted and re- 

 pelled, and that it had begun to be acted on at 

 the distance of ith of an inch from the edge of 

 the knife. The path of the ray, in passing the 

 knife edge, was bent in opposite directions, so as 

 to form a serpentine line, convex and concave, 

 towards the knife, according to the repulsive or 

 attractive forces which acted at different dis- 

 tances. In his opinion, the refraction, reflec- 

 tion, and inflection of light are all produced by 

 the same force differently modified, and do not 

 arise from the collision of light with the particles 

 of bodies. 



The great improver of achromatic telescopes, 

 after Dollond, was Fraunhofer. He observed, 

 in examining the different-coloured rays of the 

 spectrum, by means of a telescope, that though 

 the coloured spaces appear not parted by any dis- 

 tinct boundaries, yet they are broken and sub- 

 divided by numerous white and black lines. He 

 reckoned altogether above 600 lines : a few occur 

 in the red ; but they are numerous in the orange, 



yellow, green, and blue. A stripe, opened by .1 

 tine white line, divides the red; other striies 

 emerge at intervals between the orange and the 

 blue, and two very broad approximating bars 

 cross the violet. Other glass prisms, and even 

 those" lilled with liquids, gave similar appear- 

 ances. It would appear from this that refraction 

 proceeds by irregular bounds, somewhat like 

 Newton's easy fits of transmission and rHlrrtion 

 We shall finish this hasty sketch of optics, with 

 noticing the discovery of what is called polarized 

 rays of lit/fit, by Mains, a French officer of 

 engineers. The double refracting properly of 

 Iceland spar, which had been so carefully ex- 

 amined by Huygens, drew also the attention of 

 Newton, who concluded that the ray which sutlers 

 the unusual or extraordinary refraction must 

 have its opposite sides affected by some virtue 

 like magnetism, which gives them a tendency to 

 polarity. This remark lay neglected for almost 

 a century. Malus had early turned his attention 

 to the more difficult problems in optics; and, 

 after his return from foreign service, he resumed 

 his favourite studies. In one of his frequent 

 visits to the observatory, during his residence in 

 Paris in the summer of 1809, he was struck with 

 the brilliant reflection of the setting sun, from 

 one of the windows of the Luxembourg palace. 

 On looking at the appearance through a prism 

 of rock crystal, which he slowly turned round, he 

 remarked, with surprise, that one of the images 

 changed regularly to brightness from obscurity. 

 Next morning he repeated his observation with 

 the same results ; and soon found that light, re- 

 flected at a certain angle from the surface of 

 glass, acquires the same character as the extra- 

 ordinary ray in the double refracting prism. 

 Water showed a similar disposition, but at a dif- 

 ferent angle of incidence. The law was traced 

 through various reflecting surfaces, and Malus 

 was prosecuting the subject with rapid success, 

 when his career was cut short by a lingering 

 disease, on the 23d of February, 1812. The 

 subject of polarization has been successfully pro- 

 secuted in this country by Herschell and Brew- 

 ster ; and on the continent by Biot, Arago, and 

 Fresnel ; but much still remains before it reaches 

 the simplicity which characterizes the other 

 branches of optics. 



HYDROSTATICS, &c. 



THE law which determines the weight of bodies 

 immersed in fluids had been discovered by 

 Archimedes, and likewise the position of bodies 

 floating on them. It was ascertained by Stevinus, 

 that the pressure of fluids is proportional to their 



