ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



225 



socket and fixed to bench or table (fig. 18) ; and (2) when mounted in 

 socket on side of containing case (fig. l ( ->)- 



Binocular Loups of Weak and Medium Magnification.* — O.Henker 

 and M. von Rohr discuss the principles which must underlie stereo- 

 scopic vision, with especial reference to the image in space. If an 

 object-point be selected, the principal rays proceeding from it must 



Fig. 19. 



first be determined. These principal rays are those which diverge from 

 the object to the centres of' the entrance-pupils of the right and of 

 the left instrument. After their passage through the instrument, these 

 rays generally leave the exit-pupils at an increased refraction-angle to 

 their corresponding axes. A space-image, in the sense of geometrical 

 optics, occurs when the directions, produced backwards, of the principal 

 rays pertaining to a selected object-point, intersect in a point of the 

 space-image. The combination of all such space-points will give the 

 true space-image of the whole object. If such a condition, incontestable 

 as a proposition in geometrical optics, be satisfied, it is, furthermore, 



* Zeit. f. Instrumentenk., xxiA (1909) pp. 280-6 (7 figs.). 



