VISION THROUGH STEREOSCOPIC BINOCULAR MICROSCOPES. 223 



may be classed with other known phenomena, and we may leave 

 to Physiology any further consideration of the point. 



If we recall once more the dioptric and the catoptric division of 

 the pencils of rays (cf. Figs. 14 16), it is evident that one eye 

 receives an image which is formed by the right half of the objective, 

 whilst the other eye receives an image formed by the left half of the 

 objective. The image-forming halves of the objective always lie, 

 with the dioptric method, on the same side of the median plane as 

 the observing eye, and with the catoptric method, under certain 

 circumstances, on the opposite side for instance, if the equilateral 

 prisms of Nachet are employed. The impression which the fusion 

 of the two images renders perceptible will of course differ according 

 to the method of division of the pencils. The dissimilarity of the 

 images formed by the right and left halves of the objective may be 

 determined by calculation, or it may be observed experimentally by 

 successively covering up the halves of the objective. Viewing, for 

 instance, a spherical starch-grain, or a minute globule of oil, the 

 marginal shadow, which by direct (axial) illumination appears in 

 the usual image equally broad, will preponderate by covering up one 

 half of the objective on the corresponding side, so that the spot of 

 light in the image lies on the same side of the median plane as the 

 effective part of the objective. The same phenomenon will occur 

 in general with every other object which acts as a convex lens. 

 On the other hand, if the object is an air-bubble, or a cavity, or if 

 it acts as a concave lens, the spot of light 

 moves under the same suppositions to the side 

 also, but in an opposite direction. The two A 

 images which the binocular Microscope forms, 

 exhibit a distribution of light and shadow as 

 represented in A and B (Fig. 122). The 

 presentation of the one or the other image to 

 the right eye or to the left, depends, with a 

 given object, upon the optical arrangement by 

 which the division of the pencils is effected. B 

 The grouping of A and B to the right or to the 

 left clearly produces, however, the stereoscopic 



, -, rlG. 122 



effect. 1 



We may have a combination which represents elevations and 

 1 We must not forget here that a half reversal by a single reflexion produces 



ihe same effect as the changing of the images. 



O 



