APPENDIX. 



491 



holding the diaphragms, polarizing aparatus, &c. To the foot, 

 X, is attached a dovetailed slide, V, in which is contained a 



Fig 263. 



prism, E,, supporting the tube, A, and the body, C. Supposing 

 that a particular object is to be viewed, by means of the two 

 milled heads, B, we draw the prism, R, out of the axis of 

 the instrument, as shown in fig. 266, we screw the object-glass 

 to the piece F, we replace the prism under the small plate, 

 and then adjust the focus by means of the tube A and the 

 fine adjustment, F. There are in G, fig. 266, two screws, 

 which, coming in contact, prevent R from coming out com- 

 pletely. The office of the prism, R, is to receive the image in 



