252 USE OF PROJECTION MICROSCOPE [Cn. IX 



crater and the negative carbon arc most sharply defined on the 

 objective hood the light on the screen will be the best attainable. 

 Occasionally, during an exhibition, it will be necessary to use the 

 fine adjustments on the arc lamp (fig. 146) to get the crater back in 

 exact alignment as the crater changes position slightly on the wear- 

 ing away of the carbons. As the carbons sometimes wear away 

 unevenly it is necessary to have a mechanism by which one carbon 

 can be moved without affecting the other, otherwise there would 

 result some one of the malpositions shown in fig. 24, 25. 



383. Specimens for projection. The specimens giving the 

 best images with the projection microscope are those which are best 

 for ordinary observation, that is those with the most definite out- 

 lines and sharpest details. They must, of course, be more or less 

 transparent. For staining, any color which gives definite details 

 can be used, but one must remember that the red colors are trans- 

 parent to the longer, visible waves of light and hence red-colored 

 objects can remain on exhibition much longer than hematoxylin, 

 osmic acid or other dark stained objects which are more opaque to 

 the long waves in the red end of the spectrum (fig. 307). 



No matter how large the water-cell or the cooling stage, a thick, 

 darkly stained specimen will be spoiled after a time by the trans- 

 formation of the absorbed light into heat ( 852). 



384. Masks for microscopic slides. The light used in pro- 

 jection is of necessity so brilliant that the scattered light from the 

 microscopic glass slide is very liable to dazzle the eyes of the 

 operator when he looks at the slide in arranging it for the projection 

 of the object or objects thereon. If one has a series for example, it 

 is very difficult to select with ease and certainty just the sections 

 that are to be shown with this scattered light in the eyes. It must 

 always be remembered, too, that a very short time seems long to a 

 waiting audience; and that it lessens their confidence in the lec- 

 turer to have too much blundering in showing the specimens he 

 wishes them to see. 



All this difficult}- can be easily avoided by properly masking the 

 preparations to be shown (fig. 143, 148). 



