PROCEEDINGS OF THE SOCIETY. 495 



distinct pictures of the same object taken stereoscopically. These 

 apparently coalesced into a single flat picture; but when the head 

 was so placed that each eye received the rays issuing from the picture 

 of the opposite side, the same effect of relief or projection was obtained, 

 as if the two pictures had been combined by means of the ordinary 

 Stereoscope. 



Now I cannot see how these facts are to be accounted for in any 

 other way, than by the admission that the pictures formed of any 

 object in relief, by the different parts of a lens of sufficiently wide 

 angle of aperture, are sensibly different in their perspectives, — as a 

 very simple construction shows that they ought to be. Thus, if we 

 were to place a hexagonal prism under a lens of three times its own 

 diameter (fig. 82), and were to take a picture of it first through the 

 central circlet only, and then 



through each one of the peri- Fig. 82. 



pheral circlets in succession — 

 all the rest of the lens being 

 stopped out — we should have 

 seven dissimilar pictures : that 

 given by the central circlet 

 showing only the hexagonal 

 top of the solid, but in its true 

 figure ; whilst that formed by 

 each of the peripheral circlets 

 would give a foreshortened view 

 of the hexagonal top, but would 

 also bring in oblique views of 

 three sides of the prism. Now, 

 in the case described by Mr. 

 Claudet, we should only receive 

 the images from the two lateral 



circlets a h ; and these " pair " so as to bring out the effect of relief, 

 because they correspond with the two dissimilar perspectives w^hich 

 would be formed upon our two retinae, if we were viewing the solid 

 prism (enlarged to its apparent size) at the ordinary visual distance. 



But, it may be asked, if this is the real state of the case, how 

 is it that we obtain anything like a distinct image of any solid 

 projecting object viewed monocularly — that image being the com- 

 posite resultant of a number of superposed pictures differing sen- 

 sibly one from another. When the object is either flat or in low 

 relief, the pictures will not sensibly differ, unless taken with a 

 lens of much wider angle than the 40° which (as I have already stated) 

 I regard as the true limit for an objective to be used with the 

 stereoscopic binocular. Now we have in Mr. Francis Galton's 

 remarkable " composite portraits," a proof that pictures even of 

 different individual men, having the same general facial proportions, 

 but expressions so different that each may be at once distinguished 

 from the other, may be blended photographically into one image 

 which would not be remarked-on as wanting in definition. And so 

 the pictures formed of such an object as a Polycystine, Eucyrtidium 



