250 PROJECTION MICROSCOPE \_CH. IX a 



by an ordinary magic lantern. Indeed the projection microscope is a 

 magic lantern with short focus objectives. One of the first uses of the 

 microscope was to throw the images of various objects on a screen so 

 they could be seen by several persons at once. The light used was 

 sunlight, hence those early projection microscopes were called solar or 

 sun microscopes. If sunlight were available at all times and could be 

 controlled, it would be universally employed ; but it is not at all times 

 available and whenever it is, a heliostat is needed to keep the light 

 fixed in a given position. Sunlight is therefore practically discarded 

 and the electric light is employed for illumination. 



399. Parts of a Projection Microscope. These are shown 

 in Fig. 204 and are as follows : 



(1) Some form of projection objective on a proper support. 



(2) A stage for supporting the object. This stage should be 

 free from the objective holder ; it should have a range of diaphragms 

 so that the largest as well as the smallest objects may be shown. The 

 stage should be freely movable so that the object may be completely 

 lighted (see Fig. 209). 



(3) There should be an attachable mechanical stage with large 

 movement (B, D. 1). 



(4) A large lamp condenser for illuminating the object. If this 

 is properly constructed and the various parts of the apparatus are 

 separately movable as here shown, no special substage condenser is 

 necessary (see Fig. 209). 



(5) An electric arc lamp for lighting. 



Fig. 204, A, B, C, D. Half tone reproductions of photographs 

 of the Projection Apparatus used in the department of Histology and 

 Embryology, Cornell University. A 10 centimeter scale was photo- 

 graphed with the apparatus in each case. 



The arc lamp and the metal bed-piece are screwed fast to a thick 

 board so that the whole apparatus may be moved sidewise or elevated 

 without getting any part out of line. 



From the weight of the blocks carrying the microscope, the stage 

 and the condenser and the width of the bed-piece no clamping screws 

 are necessary for holding the blocks in place even when the board is 

 considerably elevated. The grooves and V's are so accurately fitted 

 however that the various blocks can be easily moved along the bed 

 piece and the centering is as accurate .in one position as in another. 



