A POPULAR ACCOUNT 



the light, after passing through the glass, 

 is reflected not by the parts of air 

 contiguous to the glass, but by the last 

 particles of the glass itself. This suppo- 

 sition, however, is scarcely intelligible, 

 and, besides, is contradicted by wetting 

 the surface of the glass, which immedi- 

 ately affects the reflection, which could 

 not be the case, if the reflection were 

 made by the parts of the glass. 



It has been shown that two kinds 

 of homogeneous light say blue and 

 red might be incident at the base 

 of a prism, with the same obliquity, 

 and so that the red should be wholly 

 transmitted, and the blue wholly re- 

 flected. In this case it seems incon- 

 ceivable, that the red rays should every- 

 where fall on pores, and the blue rays 

 on solid parts. A similar difficulty at- 

 tends the explication of all the pheno- 

 mena of thin plates explained in the 

 last chapter, for we must there suppose, 

 that the transmitted rays, being all of 

 certain species, meet only with pores 

 while the rays of the complemental co- 

 lours encounter nothing but solid parts. 



Still greater difficulties attend the 

 explanation of the phenomena of reflec- 

 tion of light at polished surfaces, ac- 

 cording to this theory. The substances 

 used in polishing glass and other sur- 

 faces, only bring the roughness of the 

 surface to a very fine grain, so that the 

 inequalities become too small to be 

 perceptible either to sight or touch. 

 Hence, since innumerable inequalities 

 exist, irregularly distributed over the 

 most polished surface, it follows, that if 

 the light were reflected by impinging on 

 the surfaces of the solid parts, it would 

 be as irregularly scattered about by such 

 a surface as by the roughest. 



(79.) Newton, therefore, considers that 

 light is reflected " not by a single point 

 of the reflecting body, but by some 

 power of the body which is evenly dif- 

 fused over its surface, and by which it acts 

 upon the ray without immediate contact." 



By a comparison of the refracting 

 powers of substances with their densi- 

 ties, Newton traced an evident con- 

 nexion between them. In all bodies, 

 whether solid, liquid, or aeriform, ex- 

 cept such as be of an unctuous or sul- 

 phureous nature, the refracting powers 

 are very nearly in the proportion of 

 their densities. A body of an unctuous 

 or sulphureous nature, compared with 

 one which is not so, has a greater rela- 

 tive refracting power than the propor- 

 tion of their densities indicates. But 



even in this case, bodies of this nature, 

 when compared with one another, have 

 refracting powers in a ratio not very re- 

 mote from that of their densities. 



The sulphureous principle, therefore, 

 seems to increase the action of bodies 

 on light ; and Newton conjectures that 

 this principle, existing more or less in 

 all bodies, may be the cause of all the 

 phenomena of reflection arid refraction. 

 " And as light congregated by a burn- 

 ing glass acts most upon sulphureous 

 bodies to turn them into fire and flame, 

 so, since all action is mutual, sulphurs 

 ought to act most upon light. For that 

 the action between light and bodies is 

 mutual, may appear from this consi- 

 deration ; that the densest bodies which 

 refract and reflect light most strongly, 

 grow hottest in the summer sun, by the 

 action of the refracted and reflected 

 light." 



This last observation of Newton has 

 been since proved incorrect. The most 

 dense bodies, if transparent and colour- 

 less, do not grow hot. However, the 

 theory which has formed the subject 

 of this chapter, and which has been 

 given substantially as Newton himself 

 has left it, must be received as matter 

 of hypothesis. To discuss its merits, 

 and compare it with subsequent expe- 

 riments and theories, would greatly 

 exceed the limits of the present treatise. 

 Our object here is to give a succinct 

 popular account of " Newton's Optics," 

 and not to discuss the relative merits of 

 the different theories which have been 

 advanced to represent the phenomena. 



CHAPTER VIII. 

 Thick transparent Plates. 



(80.) THE colours produced by the 

 reflection and transmission of light by 

 thin transparent plates are not pecu- 

 liar to these, but may also be exhibited, 

 under certain circumstances, by plates 

 of considerable thickness. By a series 

 of experiments with concave glass re- 

 flectors, silvered on the convex side, 

 Newton made these phenomena mani- 

 fest, and applied his hypothesis of the 

 fits of easy reflection and transmission 

 successfully, in accounting for them, 

 and in reducing them to the laws which 

 were explained in the sixth chapter. 



Before we proceed to describe these 

 experiments, it is necessary to remind 

 the reader of the several effects which 

 a beam of light undergoes when it en- 

 counters the surface of a transparent 



