4 REPORT — 1853. 



obviously surfaces of inner crystalline forms of exactly the same shape as the entire 

 crystal, from which the author drew inferences as to the original growing of the 

 crystal. He also entered into an examination of some of the optical peculiarities of 

 these crystals. 



On the Angle to be gwen to Binocular Photographic Pictures for the Stereo- 

 scope. By A. Claudet, F.R.S. 



Mr. Claudet's paper, which was illustrated by several stereoscopic Daguerreotype 

 pictures, went to establish some rules for the angle at which the photographic pic- 

 tures must be taken in order to produce, without exaggeration, the best effect of 

 relief and distance. The angle depended on the size we wished to give to the model 

 and the distance at which we were looking at it, so that pictures taken at the 

 same angle might produce different effects of relief and distance if they were exa- 

 mined more or less amplified, and the converse might produce a contrary effect. To 

 exemplify the relation between the stereoscopic effect and the dimensions of the 

 image, Mr. Claudet observed, that when we look at objects with a double opera-glass, 

 which magnifies say four times, we have four times less relief and less distance than 

 ■when we look at them with the naked eye ; and that when we turn the opera-glass, 

 looking through the larger end, we diminish considerably the dimensions of objects 

 and increase considerably the relief and distances. Mr. Claudet entered into some 

 considerations of the principles of binocular vision, in order to explain the causes of 

 perfect vision, with relief and solidity, which we obtain with two eyes. The photo- 

 graphic image being the representation of two different perspectives, we must, when 

 we look at them in the stereoscope as we do in looking at the natural objects them- 

 selves, converge more or less the axis of the eye according to the plane on which 

 the objects are represented or really situated. Therefore we make the same effort, 

 and we have the same sensation, when we look at photographic pictures, as when 

 ■we look at the objects represented. When we look at a single picture with two 

 eyes, we have less relief and less distance than when we look at the same picture 

 ■with one eye, because with one eye we have the natural effect we are accustomed to 

 feel when we look at the natural objects with one eye, while, if we look at the single 

 picture with two eyes, we have on the two retinae the same image with the same 

 perspective, which is not natural, and the eyes have not to make the usual 

 effort for altering their convergence according to the plane on which the object 

 observed is situated. This inaction in the convergence of the eyes destroys in some 

 measure the illusion of the picture, because the same convergence for all the objects 

 represented gives an idea that they are placed on the same plane. The angle of the 

 two binocular photographic pictures may be larger than the natural angle of vision, if 

 ■we suppose that the reduced model is examined at a small distance. If we have 

 before us, at 2 or 3 feet, a model of our friends of the size they appear to be at 20 

 or 30 feet, we have a greater effect of relief than if we were looking at them at 20 

 or 30 feet, and this effect, instead of being a defect, is more artistic and satisfactory. 

 We may reduce the model as much as we like, and look at it at the smallest distance 

 possible ; but in order to preserve the proper proportion between the stereouropic 

 effect of the nearest and furthest planes, we must take the photographic pictures 

 with object-glasses having the longest focus possible. It is only when we employ, 

 too, lenses of short focus that the stereoscopic effect is unnatural, being exaggerated 

 for the more distant planes and reduced for the nearest. 



On the Practice of the Daguerreotype. By A. Claudet, F.R.S. 



This morning was devoted to Photography ; the Section having requested Prof. 

 R. Hunt and Mr. Claudet to arrange the means of exemplifying all the processes at 

 present employed. By the aid of the local photographic artists, this was accomplished 

 in as satisfactory a manner as the suddenness of the occasion would admit of. Mr. Hunt 

 explained all the processes on paper and on glass, while Mr. Claudet exhibited the 

 manipulatory details of the Daguerreotype. A great number of very beautiful speci- 

 mens of the art were exhibited. Two views in particular, executed by Messrs. Ross 

 and Thomson of Edinburgh, of an unusually large size, were most remarkable for 



