THE STEREOSCOPE. 



I4E 



when the eyes are fixed, subtends an angle of about 180, because beyond 

 that the rays cannot enter the pupil of the eye. But in the lower animals 

 the fish and birds, notably the ostrich, the field of vision is much more 

 extensive, because the pupils are more prominent, or the eyes are set more 

 towards the sides of the head. The ostrich can see behind him, and fish 

 can see in any direction without apparent limit. Man can only see indis- 

 tinctly; and though he can move his eyes rapidly, he can see distinctly but 

 a small portion of any object at a time, yet he sees with both eyes simul- 

 taneously a single object, because the two lines of vision unite at a single 

 point, and as the two images cover each other we perceive only one image. 

 Beyond or within this point of meeting the vision is indistinct, but the angle 

 of convergence is always varied according to the distance of the object. If we 

 hold up a penholder in front of us, and in a line with any other defined object, 

 say an ink-bottle, 'we can see the penholder distinctly, and the ink-bottle 

 indistinctly, as two images. If we then look at the ink-bottle we shall see it 

 single, while the penholder will appear double, but with imperfect outlines. 



Fig. 147. The Stereoscope. 



Fig. 148. Mode of taking photograph for Stereoscope 



Again, if we look at a box both eyes will see it equally well, but the right 

 eye will see a little more on its right side, and the left eye on the left. It is 

 on this principle that the Stereoscope is constructed. Sir Charles Wheatstonc 

 was the inventor, and the instrument may be thus described : Two pictures 

 are taken by photography one as the landscape is seen by the right eye, 

 the other as it is viewed by the left ; the points of view thus differing 

 slightly. When both eyes are simultaneously applied to the instrument the 

 view is seen exactly as it would appear to the beholder at the actual place 

 it represents. The views are taken singly ; one side at one time, and another 

 after, as in the camera (fig. 148). A is the first view; B is kept dark; C is the 

 shutter for A. There are Reflecting and Refracting Stereoscopes. In the 

 former a mirror reflects the image into each eye ; in the latter the views are 

 pasted on a card side by side, and looked at through prismatic lenses. The 

 principles of Binocular Vision have been applied to the Microscope. 



In foregoing chapters we have given many examples with diagrams of the 

 temporary impressions made upon the retina of the eye. It is a fact that a 

 wheel revolving at a great rate will appear to be standing still when suddenly 



