CH. IX] 



PROJECTION MICROSCOPE 



265 



Coplin, the operator can work with rapidity, certainty and also with 

 comfort. (Fig. 213.) 



5_ 

 10 



T- 



79 



86 



93 



"1 



Homo 



5 



Slide 



10 



Sec' 8 



79 



20m 



100 



1901 



Fig. 213. Slide of serial sections with a black mask, perforated over the sec- 

 tions to be demonstrated with the projection microscope. This mask is put on the 

 back of the slide, not on the cover-glass. 



Unless one has a mask something like this the light is so dazzling that it is 

 almost impossible to find the proper sections. It is easily removed by placing the 

 slide on wet blotting paper. 



§ 426. How to demonstrate with the Micro-Projection Ap- 

 paratus. — Microscopical preparations are not so easily used as are lan- 

 tern slides. The writer has found that the most successful method is 

 for the teacher himself to stand by the apparatus, insert the specimens, 

 and find exactty what he wishes his pupils to see. Then to point 

 them out a bamboo fish pole with sharp end is used. This should be 

 2-3 meters long and if held out in the diverging cone of light leaving 

 the microscope, a sharp shadow will be cast upon the image. With 

 this pointer one can indicate the part to be demonstrated even more 

 satisfactorily than as if he pointed them out directly on the screen. 

 While it is not possible to delegate the finding of the specimen to an 

 assistant he is of great help in keeping the carbons of the lamp in ex- 

 actly the right position. If the light is kept perfect the teacher has 

 very little trouble with the rest of the manipulation. 



§427. Cleaning the Glass Surfaces of the Micro-projection 

 Apparatus. — Inasmuch as it is so difficult to make the light sufficiently 

 brilliant for micro-projection, it is of the greatest importance that all 

 glass surfaces be kept as clean as possible. The lenses of the lamp 

 condenser should be carefully wiped occasionally ; and the water bath 

 should be opened and the plane glass faces thoroughly cleaned. It 

 is desirable to soak them in the cleaning mixture for glass. There is 

 always a certain amount of deposit on the glass even though distilled 

 water is used. Every grade of opacity renders the image on the screen 

 less excellent. Cleanliness is one of the most important requirements 

 for successful micro-projection. 



