CH. IX a] PROJECTION MICROSCOPE 259 



FIG. 208 A. H. Diagrams of the stage and its parts, its grooved block and 

 the bed-piece with Vs. Half actual size. 



A. Large square diaphragm fitting the still larger square diaphragm B. ; C. 

 Vertical stem fitting into the socket formed in part by the grooved block D. ; E. 

 Bed-piece with V's which fit the grooves of the blocks carrying the various parts, 

 i. e. microscope, stage and condenser (compare Fig. 204}. The bed-piece is like a 

 lathe bed and is of cast iron. The grooves and V's are accurately worked out on a 

 machine in Sibley College. 



F. Diaphragm with circular opening ; G. Specimen cooler in position ; H. 

 sectional view showing the thickness of the stage and groove, and V, which serve 

 to hold the diaphragms in position. 



this it is necessary to have the stage supplied with changable dia- 

 phragms to accommodate any specimen which one may desire to pro- 

 ject. This has been simply done in the apparatus shown in Fig. 204. 

 The thickness of the stage, the diaphragms and the methods of secur- 

 ing the diaphragms are indicated in Fig. 208 H. With this apparatus 

 (Fig. 204 B and Fig. 208) objects as large as lantern slides may be 

 projected. 



409. Objective Support and Focusing Arrangement. 

 This should be independent of the stage and, for all but the lowest 

 objectives, should have fine and coarse adjustment as in the best 

 microscopes. As the instrument is horizontal the fine adjustment 

 should be strong enough to move the apparatus without the aid of 

 gravity. As with the stage, this part should be freely movable along 

 the axis of the whole apparatus so that it may be put in the correct 

 position for high and for low powers and the coarse and fine adjust- 

 ment used only for the final focusing. It must also be adjustable 

 vertically so that it may be accurately centered. 



A triple or quadruple nose-piece is desirable so that one may turn 

 quickly from one power to another. Behind the nose-piece should be 

 a blackened metal screen about 15 cm. in diameter to cut off stray 

 light. The tube should be large and short so that it may not restrict 

 the field (2 cm. in diameter and 10 cm. in length). 



There should be a draw-tube with proper attaching screw so that 

 it can be put in place when oculars or other apparatus needing the 

 draw-tube are to be employed. 



410. Support for very Low Power Projection. For pro- 

 jecting objects of 40 to 60 millimeters in diameter a photgraphic or 

 planar objective of about 100 mm. focus answers admirably. This 

 should be mounted on a separate support and properly centered. Then 



