ON PHYSICAL OPTICS. 437 



of any one star must be different in quantity and in direction; it never exceeds 

 20 seconds each way, and must, therefore, in common observations, be 

 wholly insensible. (Plate XXIX. Fig. 418.) 



The quantity of light, which is reflected by a substance of any kind, 

 depends not only on the nature of the substance, but also on the ob- 

 liquity of its incidence: and it sometimes happens, that a surface, which 

 reflects a smaller portion of direct light than another, reflects a greater 

 portion when the light falls very obliquely on its surface. Bouguer found 

 that the surface of water reflected only one fifty fifth part of the light fall- 

 ing perpendicularly on it, that of glass one fortieth, and that of quicksilver 

 more than two thirds: but when the obliquity was as great as possible, the 

 water reflected nearly three fourths of the incident light, and the glass about 

 two thirds only. 



Of the light which passes by a dense substance of any kind, the greatest 

 part pursues its course undisturbed, but there is always a certain divergence^ 

 which has been called by Grimaldi diffraction, and by Newton inflection. 

 This effect is usually attended by the production of colours, and will 

 therefore require to be more particularly considered hereafter. 



The separation of colours by refraction is one of the most striking of all 

 optical phenomena. It was discovered by Newton that white light is a com- 

 pound of rays of different kinds, mixed in a certain proportion, that these 

 rays differ in colour and in refrangibility, that they constitute a series, which 

 proc'eeds by gradual changes from red to violet, and that those substances 

 whifch appear coloured when placed in white light, derive their colours only 

 from the property of reflecting some kind of rays most abundantly, and of 

 transmitting or extinguishing the rest. Dr. Herschel has added to this series 

 rays of heat less refrangible than the red, and Hitter and Dr. Wollaston have 

 discovered, beyond the violet, other still more refrangible rays, which blacken 

 the salts of silver. 



It has generally been supposed, since the time of Newton, that when the 

 rays of light are separated as completely as possible by means of 

 refraction, they exhibit seven varieties of colour, related ■ to each 



T. 



