NATURAL PHILOSOPHY. 147 



IMPROVEMENT OF THE STEREOSCOPE. 



Mr. A. Claudet has read to the British Association a paper 

 "On the Means of Increasing the Angle of Binocular Instruments, 

 in order to obtain a Stereoscopic Effect in proportion to their Mag- 

 nifying Power." In a paper on the Stereoscope, which Mr. Claudet 

 read before the Society of Arts in the year 1852, alluding to the 

 reduction of the stereoscopic effect produced by opera- glasses on 

 account of their magnifying power, he stated that, in order to redress 

 that defect, it would be necessary to increase the angle of the two 

 perspectives. This he proposed to do by adapting to the object- 

 glasses two sets of reflecting prisms, which by the greater separation 

 given to the two lines of perspectives, would reflect on the optic axes 

 images taken at a greater angle than the angle of natural vision. 



Such was the instrument that Mr. Claudet submitted to the British 

 Association to prove, as he always endeavoured to demonstrate in 

 variow memoirs, that the binocular angle of stereoscopic pictures 

 must be in proportion to the ultimate size of the pictures on the 

 retina, larger than the natural angle when the images are magnified, 

 and smaller when they are diminished ; which, in fact, is nothing 

 more than to give or restore to these images the natural angle at 

 which the objects are seen when we ajiproach them or recede from 

 them. For magnifying or diminishing the size of objects is the same 

 thing as approaching them or receding from them, and in these cases 

 the angles of perspective cannot be the same. Mr. Claudet showed 

 that, looking at the various rows of persons composing the audience, 

 with the large ends of the opera-glass, all the various rows appeared 

 too close to one another, that there was not between them the distance 

 which separates them when we look with the eyes alone ; and he 

 showed also that, with the small end, the distance appeared consi- 

 derably exaggerated. But, applying the sets of prisms to the opera- 

 glass in order to increase the angle of the two perspectives, then 

 looking at the audience as before, it appeared that the various rows 

 of persons had between them the natural distance expected for the 

 size of the image or for the reduction of the distance of the objects. 

 By applying the two sets of prisms before the eyes without the 

 opera-glass, it was observed, as was to be expected, that the stereo- 

 scopic effect was considerably exaggerated, because the binocular 

 angle was increased without magnifying the objects. But looking 

 ■with the two sets of prisms alone at distant objects, the exaggera- 

 tion of perspective did not produce an unpleasant effect. It appeared 

 as if we were looking at a small model of the objects brought near 

 the observer. By the same reason, stereoscopic pictures of distant 

 objects (avoiding to include in them near objects) can advantageously 

 be taken at a larger angle than the natural angle, in order to give 

 them the relief of which they are deprived as much when we look at 

 them with the eyes, as when <we look only with one eye ; instead of 

 being a defect, it seems that it is an improvement. In fact, the 

 stereoscope gives us two eyes to see pictures of distant objects. 



K 2 



