ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
471 
view too great an approach of C to A, B, i.e. a relative flattening of 
the corporeal image, diminished plastique ; reduction of the angle, on 
the contrary, would effect too great a separation of C from A, B, i.e. 
exaggerated plastique. 
The points of an optical system from which the views of objects 
indifferently proceed are well known by Listing’s name of “junction- 
points” ( KnotenpunJde ), and they stand to one another in the relation 
of image and object. Therefore the simplest expression of Greenough’s 
condition for orthomorphy is as given above, viz. “ Entrance-pupils, &c.” 
Fig. 77. 
From this the formula Y = — can he easily deduced ; for — = — — 
(by similar triangles). . r 
A system of Porro prisms is used ; and, in order to avoid the derange- . 
ment incidental to the lengthening of the tube for adaptation to different 
eyesights, a rotation of the tube is employed, which causes an eccentric 
rotation of the oculars, and allows wide limits of adaptability. (See 
fig. 76.) 
As stops are usually placed near the object, and for many purposes 
would be a hindrance, they are here adapted for unscrewing. But for 
the attainment of orthomorphy they are quite inadmissible, as a com- 
