342 Mr. J. Bridge on the Oblique Aberration of Lenses. 



time of oscillation is only true of wave motion ; and as long as 

 the luminous ray exists in this form, the time of oscillation is 

 unchanged. When, however, it is absorbed by a medium, the 

 particles of which are thereby set in vibration, the wave motion 

 passes into a pendulum motion, and the law of unchangeability 

 ceases to be true. The molecular motions, however, communi- 

 cate themselves to the aether, and a new wave motion is thus 

 generated, which, as regards the time of oscillation, is different 

 from that of the incident light, but the same as that of the pen- 

 dulum motion from which it originates. 



As before remarked, we have long possessed in the absorption 

 of calorific rays an example of a similar change. Between the 

 dispersed light and the absorbed heat there is, however, an im- 

 portant difference ; the former ceases with the illumination, the 

 latter not. What can be the cause of this difference ? It is dif- 

 ficult to answer with certainty. But the reason is probably to 

 be sought in the circumstance, that the substances under exami- 

 nation do not possess any capacity of conduction for colours 

 eimilar to their conductivity for heat; so that the molecular 

 motions of the medium are immediately transferred to the aether, 

 and become dispersed in the real sense of the term. It is also pos- 

 sible that the molecular motions produced by the action of light 

 are not infinitely small, and hence, in accordance with what has 

 been adduced, a higher order, such as the octave, may be com- 

 municated to the aether-vibrations as long as the body is illu- 

 minated, but that on the withdrawal of the latter they become 

 insensible. In such a case the medium would, however, have its 

 temperature increased. 



Stokes also assumes that the molecular motions of the absorb- 

 ing medium are not infinitely small, but he draws from this the 

 conclusion, that the periods of motion of the medium are longer 

 than the aether waves which produce them, while according to 

 my investigation exactly the opposite must be the case ; pre- 

 mised, moreover, that the parts of the medium swing isochronous 

 with the aether, and not in conformity with its own elasticity. 



XLIX. On the Oblique Aberration of Lenses. 

 By John Bridge, M.A. University College, London*. 



THE importance of destroying oblique aberration in the con- 

 struction of photographic lenses has induced me to attempt 

 the problem. This I have done in the manner which seemed 

 most suited to the purpose, which was to ascertain the point 

 where a given ray of a pencil emerging from any point of a flat 



* Communicated by the Author. 



