CH. VIII} PHOTO-MICROGRAPHY 229 



Preparations in animal histology must approximate as nearly as possible to the 

 conditions more easily obtained with vegetable preparations. That is, they must 

 be made so thin and be so prepared that the cell outlines will have something of 

 the definiteness of vegetable tissue. It is. useless to expect to get a clear photo- 

 graph of a section in which the details are seen with difficulty when studying it 

 under the microscope in the ordinary way. 



Many sections which are unsatisfactory as wholes, may nevertheless have parts 

 in which the structural details show with satisfactory clearness. In such a case the 

 part of the section showing details satisfactorily should be surrounded by a delicate 

 ring by means of a marker (see Figs. 61-66). If one's preparations have been 

 carefully studied and the special points in them thus indicated, they will be found 

 far more valuable both for ordinary demonstration and for photography. The 

 amount of time saved by marking one's specimens can hardly be overestimated. 

 The most satisfactory material for making the rings is shellac colored with lamp- 

 black. 



Ten years ago many histologic preparations could not be satisfactorily photo- 

 graphed. But now with improved section cutters, better staining and mounting 

 methods, and with the color screens (§ 356) and isochromatic plates (§355) almost 

 any preparation which shows the elements clearly when looking into the micro- 

 scope can be satisfactoril} 7 photographed. Good photographs cannot, however, be 

 obtained from poor preparations. 



I 369. Light. — The strongest available light is sunlight. That has the defect of 

 not always being available, and of differing greatly in intensity from hour to hour, 

 day to day and season to season. The sun does not shine in the evening when 

 many workers find the only opportunity for work. Following the sunlight the 

 electric light is the most intense of the available lights. Then come magnesium, 

 acetylene, the lime light, the gas-glow or Wellsbach light, and lastly, petroleum 

 light. The last is excellent for the majority of low and moderate power work. 

 And even for 2 mm. homogeneous immersion objectives, the time of exposure is 

 not excessive for many specimens (40 seconds to 3 minutes). This light is also 

 cheapest and most available and has the advantage of being somewhat yellow, and 

 therefore in many cases makes the use of a color screen unnecessary if one uses 

 isochromatic plates. Acetylene light is excellent and may be used where the arc 

 light is not available. 



A lamp with flat wick about 40 mm. ( \% in. ) wide has been found most gen- 

 erally serviceable. For large objects and low powers the flame may be made 

 large and the face turned toward the mirror. This will light a large field. For 

 high powers the edge toward the mirror gives an intense light. The ordinary 

 glass chimney answers well, especially where a metal screen is used as shown 

 in Fig. 184. 



EXPERIMENTS IN PHOTO-MICROGRAPHY 



§ 370. The following experiments are introduced to show prac- 

 tically just how one would proceed to make photo-micrographs with 

 various powers, and be reasonably certain of fair success. If one con- 

 sults prints or the published figures made directly from photo-micro- 

 graphs it will be seen that, excepting the bacteria, the magnification 

 ranges mostly between 10 and 150 diameters. 



