THE PSET7DOSCOPE. 287 



So wonderfully close is the representation, that the idea of a 

 place obtained by means of the combination of the photograph 

 and Stereoscope is quite as vivid and correct as if it had been 

 gained by actual observation. 



The principle of the Stereoscope is now applied to the best 

 microscopes, and its value is incalculable, especially when low 

 powers are used, i.e. those of not less than half an inch focus. 

 The real beauty of many objects could never have been appre- 

 ciated but for this discovery, nor their true form defined. 



On the left hand of the illustration is shown the combining 

 power of the eyes. Supposing the right eye only to be brought 

 to bear upon the little cylinder, only one side of it will be 

 seen, and it looks nearly flat. The same is the case with the 

 left eye. But, when both eyes are used together, both sides of 

 the cylinder are presented to the mind, and thus we get the 

 effect of solidity. 



The Stereoscope is so formed, by means of lenses, that the 

 two figures become combined into one, the rays of light being 

 turned out of their course by the arrangement of the glasses. 



The Stereoscope, however, although a useful assistant to the 

 vision, is not necessary. It is perfectly possible to combine the 

 two figures without any stereoscope, and to do so merely by 

 squinting, if we may so call it, at the figures. The power of 

 combination is gained with a very little practice, and in a 

 short time the observer will be capable of producing stereo- 

 scopic effects without needing a Stereoscope. This ability is 

 very useful when inspecting photographs in a shop-window. 

 Of course the figures are not so much enlarged as they are 

 with the stereoscope, but they are nevertheless quite as clear 

 and well defined. 



THERE is an instrument called the Pseudoscope, which, as its 

 name imports, gives a false idea as to the nature of the object 

 which is viewed through it, converting hollow objects into 

 solid, and vice versa. The following description of its effect is 

 given by Wheatstone : 



" "When an observer looks with the pseudoscope at the interior 

 of a cup or basin, he not unfrequently sees it at first in its real 

 form ; but by prolonging his gaze he will perceive the conver- 

 sion within a few minutes; and it is curious that, while this 



