138 Methods in Plant Histology 



the tube well blackened inside. Move the light back and forth and 

 sidewise to get the best illumination. About six inches from the 

 stage is likely to be somewhere near the proper position. If the 

 illumination is still uneven, remove the condenser from the micro- 

 scope. The ordinary form of Abbe condenser is not likely to be 

 satisfactory with -objectives of 16 mm. focus and should not be used 

 at all with objectives of such long focus. 



Focus the object upon the ground glass. Even with a 16 mm. 

 objective, the ordinary ground glass is rather coarse for accurate 

 focusing. Always examine the image with a focusing lens. A 

 brilliant view may be obtained by fastening a thin cover-glass to the 

 ground glass with a small drop of balsam. At this spot the image 

 may be examined very critically. Of course, as in any photography, 

 the ground side of the glass should be nearest the object, occupying 

 exactly the place which is to be occupied by the emulsion side of the 

 plate. Do not focus indiscriminately, but be sure that the image 

 is sharp at the level of the ground side of the glass. It is a good plan 

 to make a cross upon the ground glass with a pencil or pen, and then 

 add a drop of balsam and a cover-glass. Focus on this mark and 

 fix the focusing glass at this level. The cheap tripod lenses are good 

 for this purpose. 



The time of exposure will vary with the magnification, the 

 intensity of the light, and the speed of the plate. The exposures will 

 be much longer than in ordinary photography. It is better to use 

 artificial light, since one can more quickly learn to estimate the 

 length of exposure when the intensity of the light is constant. A 

 slow plate, even the very slow contrast plate, is likely to prove most 

 satisfactory. With a Welsbach lamp, a contrast plate of the same 

 speed as a lantern slide, and a 16 mm. objective used without an 

 ocular, or Abbe condenser, try an exposure of 30 seconds. Develop 

 the negative in whatever solution is recommended in the directions 

 which come with every box of plates. If the negative is too weak, 

 make a longer exposure; if too dense, shorten the exposure. A 

 little experience with your apparatus will soon enable you to estimate 

 the length of exposure with some certainty. We use lantern slides 

 for all tests and for small photomicrographs. The Cramer lantern 



