ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



689 



Figp, 78 and 79 will serve to illustrate the two positions of the 

 Microscopes T and T,, when adjusted first to the surface of the reflector 

 A B (H being the " principal point ") and afterwards to its centre C, 

 and Oi being the objectives ; d the eye-piece of the Microscope, and 

 d, the diaphragm in the other, and a a^ the jioints in the plane of the 

 centre of curvature to which the Microscopes are directed in the second 

 position. 



The complete instrument with the two Microscopes is shown in 

 fig. 80. It is fixed on a cast-iron table T which also supports F, Q, R 

 for holding the head of the patient in the required position. Each 

 Microscope has a tube for the objective o, and an inner eye-piece-tube 



Fig. 78. 



Fig. 79. 



d and d'. In the right tube the eye-piece is replaced by a small round 

 plane mirror from the centre of which the silvering is removed in the 

 form of a cross. The left tube has a Huyghenian eye-piece, with cross- 

 threads. Both objectives are of equal power with a focal length of 40 

 mm. The Microscopes are placed in two short tubes /* h^ attached to 

 supports s Si which rest on the plate P. In the latter are guides, which 

 regulate the movements of the tubes, so that they can only move in the 

 direction of their axes. For disjilacing the tubes an eccentric movement 

 is employed consisting of a cylindrical steel axis A, with a radius of 

 12 mm., protected by a brass sheath H attached to the stand, so that 

 it lies horizontal and immediately under the plate P, at right angles 

 with the vertical plane bisecting the angle between the tubes. At the 

 sides, semi-cylindrical portions I Z, are removed, in order to give room 

 for the lowering of the triangular pieces 1 1^ connected with the tubes. 

 From the apices of these pieces is cut a slit at right angles to their base 

 in which slides (pressed with a spring) a cylindrical steel rod r, 4 mm. 

 thick, which lies within the periphery of A, parallel to it and at a dis- 

 tance of 10 mm. This distance can be regulated by four screws Cy Ca c^ c^. 

 If the axis is revolved on its centre by the handle X, the rod will 



