September 14, 1906.] 



SCIENCE. 



343 



ninety degrees apart. The stage-plate, f, is 

 screwed firmly to the shaft and collar a^ with 

 the stage-openings precisely in the same radii 

 with the four holes in the collar a. 



ject which is now centered in the finder will 

 be centered when rotated into the axis of the 

 projector. Hence, while the first object is 

 being projected on the large screen, the op- 



ts 



optioai - 



13 Axle- 



FiG. 1. Side Elevation 



7K ' O jJ U u dl — f ula oi* ■ 



1^ ) \ Projootaii - 



of ) \ Flndoj 



^ 



Qptloal — Axl e 



Fig. 2. Plan of base level. 



Fig. 3. 



Fig. 4. 



Fig. 5. 



To operate the finder, it is first made par 

 central, as follows: A slide bearing fine cross- 

 lines, such as a stage micrometer, is accurately 

 centered under the objective, F, of the pro- 

 jector. Then the stage is rotated 180°, and, 

 without moving the slide, the image is brought 

 to center on the screen E by adjusting the 

 lens D and the screen E. Evidently, any ob- 



erator may be finding the part of the third 

 object which he wishes to show, the second 

 having been located already. When the sec- 

 ond is to be thrown upon the screen, the 

 plunger is disengaged by means of the thumb- 

 block t, and the stage is rotated 90°, when the 

 plunger automatically locks the mechanism in 

 the proper position. Slides of uniform thick- 



