342 LECTURE XXXVII. 



This aberration is produced by the effect of the revolution of the earth 

 in its orbit, combined with that of the progressive motion of light. Since 

 light proceeds always in right lines, when its motion is perfectly undis- 

 turbed, if a fine tube were placed so as to receive a ray of light, passing 

 exactly through its axis when at rest, and then, remaining in the same 

 direction, were moved transversely with great velocity, it is evident that 

 the side of the tube would strike against the ray of light in its passage, 

 and that in order to retain it in the axis, the tube must be inclined, in the 

 same manner as if the light, instead of coming in its actual direction, had 

 also a transverse motion in a contrary direction to that of the tube. The 

 axis of a telescope, or even of the eye, may be considered as resembling 

 such a tube, the passage of the light through the refracting substances not 

 altering the necessary inclination of the axis. In various parts of the 

 earth's orbit, the aberration of any one star must be different in quan- 

 tity 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 obliquity 

 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 quick- 

 silver more than two thirds : but when the obliquity was as great as possi- 

 ble, 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 

 compound 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 proceeds by gradual changes from red to violet, and that 

 those substances which 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. Herschelt 

 has added to this series rays of heat less refrangible than the red, and 

 Hitter J and Dr. Wollaston have discovered, beyond the violet, other still 

 more refrangible rays, which blacken the salts of silver. 



* Ph. Tr. 167J, vi. 3075 ; vii. 4059, 4087, 5004, 5012, 5084 ; viii. 6086, 6108, 

 &c. &c. Opuscula, ii. 181. 



f On Heat and Light, Ph. Tr. 1800. 



J Gilbert's Ann. vii. 527 ; xii. 409. 



Ph. Tr. 1802, p. 365. See also Scheele on Air and Fire (trans.}, Loud. 1780, 

 66. 



