50 THE USE OF THE MICROSCOPE 



and can readily be increased by approximating the 

 eyepieces. 



The advantages of stereoscopic vision are the following: 

 For objects nearly in one plane, which is the case with the 

 highest magnifications, there is not much stereoscopic 

 effect to be gained. But, with lower magnifications, the 

 impression of solidity helps more or less in making out the 

 true shapes and spatial arrangements of objects. Stereo- 

 scopic vision, even when exaggerated in depth by the 

 microscope, as it always is, supplies plastic images which 

 seem more natural than flat pictures, and appear to be 

 more easily remembered. Such solid pictures lessen the 

 separation between the world of the microscope and the 

 outer world of the unaided eye. Stereoscopic vision, of 

 course, allows dissections and other manipulations to be 

 practiced with somewhat more facihty than monocular 

 or non-stereoscopic binocular vision. On the whole, how- 

 ever, a one-eyed observer through the microscope is 

 not seriously handicapped, especially for high-power 

 microscopy. 



Non-stereoscopic Binocular Vision. — The advantages 

 of binocular vision without strong stereoscopic effect, 

 when the two fields of the microscope are identical, or 

 nearly so, are as follows: The two eyes converge equally, 

 that is, their axes are parallel; the two retinas receive equal 

 amounts of light, and so the irises are equally contracted; 

 the accommodation of the two eyes is the same; the 

 floating fibers {muscce volitantes) of the two eyes do not 

 coincide in the two fields, and so are almost completely 

 cancelled; and the loss of field caused by the bhnd spots 

 of the two eyes when they enter into the field of view is 

 also cancelled. This last may be perceptible and annoying 

 when searching shdes with the monocular microscope; 

 especially if the sUde is moved up and down (instead 

 of from right to left), as it is when using the Detto slide bar. 

 The natural form of vision, with two eyes instead of one, 

 is also more comfortable; and better work can usually 

 be done with the binocular than with the monocular. 



