NOTES. 303 



Those have been ascribed by most philosophers to other causes 

 than a different reflexibility. But one proposition maintained 

 in those papers has been generally admitted to be well founded, 

 the non-existence of reflexibility in the sense of Sir I. 

 Newton ; namely, the disposition to be reflected, and not trans- 

 mitted. He conceived, that the most refrangible rays were 

 also in this sense the most reflexible ; and I ventured to make 

 this objection, that his experiment introduces different refran- 

 gibility, being the reflexion from the base of a prism after the 

 rays have been refracted, and when they are about to be re- 

 fracted again. Professor Prevost, of Geneva, impugned my 

 doctrine upon this subject in a paper which was inserted in 

 the Phil. Trans, for 1799. But it is generally admitted that 

 his objection applied rather to the course of my reasoning than 

 to the support of the Newtonian doctrine, that is, to different 

 reflexibility in the Newtonian sense ; and this, there is reason 

 to believe, has now been given up. Indeed, one circumstance 

 appears sufficient to show that it can have no existence. If, 

 instead of a prism which introduces different refrangibility 

 into the experiment, we take an extremely thin plate of glass, 

 and incline it in the rays of the spectrum, we find that there 

 is no difference in the angle at which the different rays are 

 reflected, instead of being transmitted. M. Arago, however, 

 independent of this circumstance, considered the Newtonian 

 different reflexibility as having been sufficiently disproved. 



In these papers of 1796 and 1797 the different inflexibility 

 of light was asserted, but not so fully proved as in these 

 Tracts VII. and VIII. The experiments and observations in 

 the Phil. Trans, for 1796 were made in 1794 and 1795, when the 

 paper was sent to the Eoyal Society. There was an anticipa- 

 tion of Photography given in the copy of the paper first sent, 

 but Sir C. Blagden considered that it referred rather to a 

 subject of Art, and it was left out in the copy subsequently 

 sent, and from which the paper was printed. According to 

 the best of my recollection, it consisted of a remark on the 

 effect of exposing a plate of ivory, stained with nitrate ol 

 silver, to the rays of the spectrum, and also on the effect 



