24 METHODS OF MICROSCOPICAL RESEARCH. 



landsman could not judge of the distance of a pass- 

 ing vessel to a few miles, nor could we form any 

 accurate idea of the size of any object emitting 

 practically parallel rays unless we had something 

 to compare it with. 



We now come to a point which has been much 

 disputed in the study of microscopy binocular 

 vision. 



The two eyes move together as a system, so that 

 we direct the two lines of regard to the same point 

 in space and consequently see but a single image ; 

 but it is possible to see two if one eye be displaced 

 a little with the finger two images are seen, while 

 if the other be displaced to a corresponding degree 

 the one image is restored. 



The value of binocular vision may be easily 

 ascertained by experiment. When a picture is 

 presented to the retina of each eye, the compound 

 picture is much brighter than when one retina only 

 is employed. 



To each point of the retina of one eye there is a 

 corresponding point in the retina of the other, and 

 impressions produced on one of these points are in 

 ordinary circumstances indistinguishable from similar 

 impressions produced on the other. 



When both retinse are similarly impressed, the 

 general effect is that the impressions are more 

 intense than when one eye only is employed ; and 

 we also get a perception of relief, that is of form in 

 its three dimensions. 



Take two A eyepieces and look through them to 

 the sky, so that two distinct circles are seen ; now 

 bring them together so that one circle overlaps the 

 other, when this overlapping bi-convex portion will 



