110 



to turn the thin edge of the semi-lenses of the two cameras in the 

 direction which will produce a bending contrary to that of the semi- 

 lenses 'of the stereoscope. 



Having shown how the lateral proportional distances of any two 

 correspondent points of the two stereoscopic pictures are the indices 

 of their perspective distances, if we were, while looking in the stereo- 

 scope, to produce a change in those proportional lateral distances by 

 sliding horizontally in a contrary direction, two pairs of superposed 

 glass photographic pictures, the objects would appear to move, not in 

 the horizontal lateral direction of that change which they naturally 

 have, but in a straight line forward and backward, as if the object 

 was approaching or receding. 



But the most curious effect of that motion would be, that the objects 

 would appear increasing in size while they were receding, and diminish- 

 ing while approaching, which we know is contrary to the rule of 

 perspective. This is another illusion entirely physiological, and the 

 cause of which may be thus explained ; while the object appears 

 moving forward and backward it remains always the same size, but 

 as we expect when it moves forward that it should increase in size, 

 and when it moves backward that it should decrease, and as it does 

 not, we feel that it is diminishing when approaching and increasing 

 when receding. 



II. " A Memoir upon Caustics." By ARTHUR CAYLEY, Esq., 

 F.R.S. Received May 1, 1856. 



(Abstract.) 



The principal object of this memoir, which contains little or nothing 

 that can be considered new in principle, is to collect together the 

 principal results relating to caustics in piano, the reflecting or refract- 

 ing curve being a right line or a circle, and to discuss with more 

 care than appears to have been hitherto bestowed upon the subject, 

 some of the more remarkable cases. The memoir contains in parti- 

 cular researches relating to the caustic by refraction of a circle for 

 parallel rays, the caustic by reflexion of a circle for rays proceeding 



