NATURAL PHILOSOPHY. 153 



round their vertical axes so that the angle between their surfaces and the 

 long edge of the box is somewhat greater than 4-3 . The objects then 

 appear greatly reduced in size, but in surprisingly prominent relief. When 

 the large mirrors only are turned, the small ones being left at the angle 

 of 4-3 , an exaggerated relief is obtained. If the dimensions in the direc- 

 tion of the depth of the field of view, to those on the surface, are to retain 

 their right relations, the large mirrors must always remain parallel to the 

 small ones. The aspects of near objects, particularly of the human figure, 

 are strikingly beautiful in the telestereoscope. The impression differs from 

 the reduction produced by concave glasses, in the circumstance that it is not 

 reduced pictures that the observer imagines he sees, but actually reduced 

 bodies. 



Magnifying power may easily be connected with the telestereoscope: it 

 is only necessary to place a double opera-glass between the eyes of the 

 observer and the small reflectors; it is still preferable, for the field of view, 

 to separate the eye-glass from the object-glass of the instrument, and so 

 fix them in the telestereoscope that the light at each side first strike the 

 large mirror, then the object-glass, then the small reflector, and finally, 

 the eye-glass; so that in this arrangement the optic axis of the telescope 

 itself is broken at a right angle. The greater the magnifying power, the 

 greater, of course, must be the perfection of the plane reflectors ; but then 

 it is not necessary to choose them larger than the object-glass of the 

 telescope. 



These views, at the same time telescopic and stereoscopic, also exceed, 

 to an extraordinary degree, the common image of the telescope in vivid- 

 ness. In the simple telescopic images, difference of distance disappears 

 totally: the objects look exactly as if they were painted on a plane sur- 

 face. By the ordinary combination of the two Galileo's telescopes, the 

 appearance of relief for nearer objects is in some degree obtained; and 

 hence it is that a double opera-glass gives a much livelier impression of 

 relief than a single one. But in the usual construction of the instrument 

 the relief is false; the objects appear as if they were squeezed together 

 in the direction of depth. In the case of human faces, on which, for the 

 most part, opera-glasses are directed, this is very striking. When they 

 are regarded from the front, they appear much flatter than they really 

 are; and when looked at in profile, they appear too narrow and sharp. 

 In both cases the expression of the countenance is essentially altered. 



When a double opera-glass is turned round, and the observer looks 

 through the object-glass, the deep dimensions of objects are magnified out 

 of proportion. While, therefore, through a simple telescope all objects 

 appear as paintings, through a double opera-glass complete objects ai*e 

 seen as bas-reliefs; while, by reversing the opera-glass, objects appear in 

 high relief. 



ON THE PHENOMENA OF RELIEF OF THE IMAGE EORMED ON THE 

 GROUND GLASS OF THE CAMERA OBSCURA. 



The following paper was recently read before the Royal Society, by M. 

 Claudet : 



The author having observed that the image formed on the ground glass 

 of the camera obscura appears as much in relief as the natural object, 

 when seen with the two eyes, has endeavored to discover the cause of 



