386 SUMMARY OF CURRENT RESEARCHES RELATING To 



form on the sensitive plate as it would Lave been on the retina of the 

 eye had the eye been substituted for the photographic objective. The 

 result is that vision instead of bearing directly on the landscape, is 

 arrested on the proof representing it : it is an image, similarly reduced, 

 of this first reduction which is impressed on the retinae. Each of the 

 latter acts separately and possesses the property of revealing in miniature 

 the landscape photographed, as the real landscape when viewed directly 

 is seen in its natural size. If a retinal image be reversed in direction it 

 will reproduce the landscape in real size with its attributes of length, 

 width, and depth ; but if such an image exteriorise itself by means of a 

 photographic proof, it will reproduce the landscape more or less reduced, 

 as the three attributes will be in reality there, although the stereogenic 

 property is for the time being suppressed. The author points out, as 

 one of his illustrations, that single-eye observation of a perfectly 

 illuminated ordinary photograph is seldom slow in detecting the details 

 in the proof in their relief and depth. The dissociation of the two retinal 

 images is then spontaneously accomplished : the two images, in fact, 

 separately appear if one fugitively opens the second eye. Two-eye 

 vision, really, brings the sensation of a plane image, and so long as 

 single-eye observation is continued, the proof exhibits stereoscopic 

 characteristics which persist if the primitive proof is replaced by a 

 numerous series of others. If, instead of focusing one's optic axes 

 directly on to a photographic proof, one makes them converge beyond it, 

 the dissociation is again obtained and the landscape is seen double with 

 all its reliefs and depths. If the focus is brought back on to the proof, 

 the images fuse and the sense of relief disappears. The process which 

 lends itself to the continuous and rapid repetition of these alternations 

 must be the one to furnish the most complete information on the 

 mechanism for the acquisition of the stereogenic property of retinal 

 images furnished by moving photographs. Hence it may with confidence 

 be declared that this acquisition is the necessary consequence of the rever- 

 sion and of the exteriorisation of these images, projected in a state of 

 dissociation outside the eye. 



Additional Demonstration of the Mechanism of Monocular 

 Stereoscopy.* — In this article A. Chauveau goes more fully into the 

 theory of his subject, and describes several experiments. He concludes 

 that the systematic use of dissociation prisms is to be recommended for 

 the demonstration of the unity of the mechanism both of monocular 

 stereoscopy and of binocular stereoscopy, both methods depending in 

 the same manner on the phenomenon of reversion and of exteriorisation 

 of retinal images. Even as regards the purely picturesque observation 

 of stereoscopic photographs, this method is just as much to be recom- 

 mended. With the two bare, prisms in general use one obtains, in 

 reality, besides the relief of the classic image of the ordinary stereoscope, 

 that of the two components of this classic image. The simultaneous 

 vision of these three images in a more or less marked relief, forms a 

 picture so much the more interesting because the observer sees it in 

 instantaneous self -constitution under his eyes, and because it explains 



* Comptes Rendus,|cxlvi. (1908) pp. 846-53 (1 fig.). 



