122 SUMMARY OF CURRENT RESEARCHES RELATING TO 
adapter between the tube and tlie objective. Light entering through a 
lateral opening in the revolving sleeve K is reflected downwards by the 
prism P, which covers half of the aperture of the objective, and is con- 
centrated upon the object by the objective. To obtain the proper 
incidence of the rays, .the fitting carrying the prism is made to rotate 
about the optic axis. The prism P can within certain limits be inclined 
about an axis parallel to its edge by means of the milled head 1c. 
Zeiss’ new Lens-holder.* — Prof. A. Zimmermann has devised the 
new lens-holder shown in fig. 18. In its external form it is very similar 
to the older Zeiss model. An important feature which distinguishes it 
from this is that the whole upper part can be rotated about a vertical 
axis, and clamped in any position by the screw a. The displacement of 
Fig, 18. 
the lens in the vertical plane is effected as in the older model by means 
of two joints b and c. The second distinguishing feature of the new 
instrument is that both joints can be simultaneously arrested with one 
screw. Figs. 19 and 20 explain the method, devised by Herr M. Berger, 
by which this is effected. In these figures the two joints b and c, with 
the arm / between them, are shown in two positions at right angles. 
The movement of the joints b and c will be so much more difficult the 
closer the joint-faces are pressed together. By means of the screw g 
such a pressure can be simultaneously applied to both joints. This is 
effected by means of the iron rod h in the tube /, one end of which is 
rigidly connected with the ball-joint c, while the ether end has a conical 
hole, in which fits the conical end of the screw g. When the screw is 
screwed farther into the joint b , the rod h is displaced to the left, and the 
* Zeitsclir. f. Instrumcntenk., xv. (1895) pp. 322-3. 
