METHODS OF RESEARCH: PHOTOMICROGRAPHS 125 



with low powers. The field must be flat, must be uniformly 

 lighted, and the proper focus must be obtained, which focus is 

 not exactly that of the eyepiece, but must be secured by use of a 

 hand lens on the ground glass, or preferably on clear glass 

 substituted for the ground glass. I first focus with the eye- 

 piece, then attach the bellows and focus on the ground glass 

 under a thick dark cloth (two folds), at which time I judge of 

 the uniformity of lighting and center my object, then I sub- 

 stitute the plain glass for the ground glass and re-focus, using a 

 hand lens. Last of all, I stop-down, always considerably, and 

 make the exposure. In all this I am considering only apochro- 

 matic objectives. With some of the older Zeiss achromatic 

 objectives I found it impossible to get a focus upon the ground 

 glass that would photograph. In this case I determined experi- 

 mentally by a series of photographs made at slightly different 

 levels the amount of the error and thereafter obtained a sharp 

 focus on the ground glass, turned it out the required amount, and 

 then made my exposure. 



Photomicrographs require a longer development than ordi- 

 nary photographs since in the latter we aim at a pleasing grada- 

 tion of tints whereas we usually have in the stained slides sharp 

 contrasts and wish to retain these contrasts on the negative. 

 We use hydroquinon developer (double the amount of hydro- 

 quinon of Cramer's formula with the metol left out) and I 

 develop until the image is well through on the back. This 

 occurs in a fully exposed plate in seven minutes, in a slightly under- 

 exposed one in 15 to 25 minutes. If I wish very marked con- 

 trasts such as deeply stained bacteria on a white background 

 (Fig. 187 for example) I under-expose so that the image comes 

 up in five to seven minutes rather than in two minutes, and I 

 develop a long time (30 or 40 minutes), with a hydrochinon 

 contrast developer, so as to have a very dense background. 



The plates are fixed in the green (chrome alum) acid fixing 

 bath where they should remain at least 20 minutes, and much 

 longer will not hurt them. We prepare the bath, only when we 

 are ready to use it, from standard stock solutions which keep 

 indefinitely separate, adding to four tumblers of the filtered 

 saturated water solution of sodium hyposulphite (water 128 



