ZOOLOGY AND BOTANY, .MICROSCOPY, ETC. 385 



simple eye, their combination recalls the compound eye of an insect. 

 The first property of such a system is to give photographic images 

 without its introduction into a camera. It suffices to present the system 

 in full light before the objects to be represented. The use of a camera is 

 unnecessary, because each cellule is itself a camera. The pellicule must, 

 of course, be preserved in a light-tight box, and only exposed as required. 

 The result is to give a series of small microscopic images fixed each on 

 the retina of one of the cellules. Observed from the side of the sensitive 

 layer, these images could not be distinguished with the naked eye, and 

 they would give the impression of a uniform grey. On the other hand, 

 suppose the eye placed on the anterior side, and the proof illuminated 

 by transparency in diffused light, such as would be furnished by white 

 paper applied to the pellicule ; the eye would then see, instead of the 

 system of small images, a single resultant image projected in space in 

 actual size. Moreover, this image will vary according to the position of 

 the observer's eye. Such an image would be a negative, but the author 

 suggests means for obtaining a positive. 



With regard to the technical difficulties, the author points out that 

 the necessary texture of surface must await the invention of a suitable 

 moulding machine of high precision. But possibly collodion and 

 celluloid could be abandoned in favour of other refrangible materials. 

 Glass, for instance, would furnish the lens-spherules in unlimited 

 quantities ; but there would still be the difficulty of sifting them out 

 with precision and affixing them on a membrane of collodion, so as to 

 obtain an exactly suitable thickness. If glass of index-refraction exactly 

 equal to 2 • could be obtained the difficulties would largely vanish, for 

 a sphere of such a glass converges parallel rays on to its posterior surface. 

 Such a sphere half-covered with a sensitive layer furnishes the simplest 

 •of cameras. Glass can be obtained with refractive index greater than 

 2*0, and also exceeding 1*9, but at present 2*0 is unattainable. The 

 mixtures of silicates with molybdates and tungstates of lead, which 

 might be expected to give the required result, seem always to crystallise 

 out without formation of glass. 



Perception of Relief and Depth in the Simple Image of Ordinary 

 Photographic Proofs : Conditions and Theory of this Perception.* 

 Lippmann's observations on integral photographs, noticed above, 

 have suggested various considerations to A. Chauveau, which he has 

 treated in a paper with the above title, and he adds that tho scope 

 of his article would be indicated by the addition of the following 

 sub-title : — The stereogenic property of retinal images, dissociated by 

 cessation of the convergence of the two optical axes on the surface of a 

 simple photographic proof. Exterior isation of these two retinal images, 

 with projection of their details on the respective planes which they occupy 

 in the depth of the space photographed. In the course of his treatment 

 the author emphasizes the principle that binocular vision is not necessary 

 to the appreciation of relief and of distances : it is capable only of im- 

 proving this appreciation. Now photographic representation of a 

 landscape is only an intermediate reception, a kind of relay between the 

 eye and the landscape. The latter, in reality, is impressed in reduced 



* Comptes Rendus, cxlvi. (1908) pp. 725-30. 

 June 17 th, 1908 2d 



