74 Royal Society : — 



concavity and convexity according to the position of their thin edges ; 

 of concavity when their edges are towards each other, and of con- 

 vexity when they are placed contrariwise. For this experiment we 

 have only to employ a pair of those spectacles mounted with a spring 

 whereby they are held on the nose. 



When we read, holding such spectacles with both hands, we may 

 by the elasticity of the spring adjust the two lenses so that the pupils 

 of the eyes can coincide, first, with the two nearest edges, secondly, with 

 the two centres, and thirdly, with the two furthest edges of the lenses. 



In the first case, the'page of the book will appear concave, because 

 the pupils will look through the thin edges of the lenses which bend 

 the vertical lines with their concave sides turned towards each other ; 

 in the second, the page will appear flat, because the pupils will look 

 through the centres of the lenses which show the vertical lines per- 

 fectly straight ; and in the third case, the page of the book will ap- 

 pear convex, because the pupils will look through the thin edges of 

 the lenses which bend the vertical lines with their convex sides turned 

 towards each other. 



These considerations have led the author to construct a stereo- 

 scope which presents flat surfaces perfectly flat. This new stereo- 

 scope has two entire lenses instead of two semi-lenses, and the eyes 

 look through the centre of such lenses. The images not being late- 

 rally refracted, as in the semi-lenticular stereoscope, their coalescence 

 requires a certain effort of divergence, or to squinting outwards, 

 which a little practice will enable us to perform easily. Persons 

 capable of using this kind of stereoscope will sec a picture whose 

 surface is perfectly flat with all the illusion of relief and distance. 



All lenses being more or less subject to the defect of bending 

 straight lines when refracted by all the various points of their surface 

 but the centre, and in a greater degree as those points are nearer the 

 edges, it residts that when images are produced in the camera ob- 

 scura by the various points of the whole aperture, they will be bent 

 in various contrary directious, and a certain confusion must arise 

 injurious to the delicacy and correctness of the whole compound 

 image. This may be proved by the following experiments: — If we 

 take the image of a window by a small aperture placed on the right 

 edge of a lens, say of 3 inches aperture, and another image of the 

 same window, by placing the aperture on the left, taking care to shift 

 the camera so that the two apertures will be exactly on the same 

 line, we shall have two images of the same window apparently iden- 

 tical ; but in placing these two images side by side in the central lens- 

 stereoscope above described, first the image of the left side aperture 

 on the right, and that of the right side aperture on the left, secondly 

 the images vice versa, we shall see in the first case a concave window, 

 and in the second a convex window. But in examining the two 

 images in the semi-lenticular stereoscope, we shall see in one case a 

 concave window, and in the other a perfectly flat window, because 

 in the first case the stereoscope will have increased the bending of 

 the vertical lines of the two images, and in the 'second the stereoscope 

 will have corrected the bending. 



