322 SCIENTIFIC AMUSEMENTS. 



Expt. 356. Stereoscopes. When an object at a moderate dis- 

 tance is looked at, the impression produced upon one eye is not 

 identical with that produced on the other, as the two or three 

 inches of horizontal difference in position of the two eyes corres- 

 ponds with a different view of the relationships of surrounding 

 objects to one another. Thus, a photograph taken at a particular 

 spot would not be identical with one obtained by moving the 

 camera a little to one side ; objects that would be directly in line in 

 the first case would not be so in the second, and vice versd. When 

 an individual in good health and possessing perfect vision looks at 

 a view, &c., with both eyes, the two slightly different impressions 

 produced on his two eyes become combined into one impression, 

 giving the idea of solidity or perspective. In certain states of 

 health, however, owing to derangement either of the optical con- 

 stituents of the eyes, or of the nerves connected with them, this 

 fusion into one of the two impressions does not always occur, so 

 that double vision results. This can generally be brought about 

 at will for a few seconds by gently pressing one of the eyeballs 

 either towards the nose or away from it, when two images of an 

 object looked at will usually be seen. Certain drugs, and more 

 particularly alcohol (when given in greater quantities than 

 certain limiting amounts varying with the constitution and habits 

 of the individual), also produce the same result of "seeing double," 

 along with others due to temporary derangements of the nervous 

 mechanism through the presence of alcohol in the system. 



If two pictures are obtained (conveniently by means of photo- 

 graphy) one representing a view of a given object from one spot 

 and the other another view of the same object from a slightly 

 different spot, and these two pictures are presented, the one to the 

 one eye and the other to the other, after a short time the two 

 impressions will blend together and a single 

 impression will result, exhibiting apparent 

 solidity. Instruments for bringing about 

 this result are termed stereoscopes. The 

 best known form of stereoscope simply con- 

 sists of a frame holding the two pictures in 

 position in such a way that the one in- 

 tended to be seen by the left eye cannot 

 be seen by the right, and vice versd (fig. 

 162); two lenses or portions of lenses are 

 usually also supplied at the top of the 



frame, one for each eye, partly for the sake 

 reoscope. Qf magnificationj partly to agsist in focus _ 



sing" the two pictures into one. Fig. 163 represents the two 



