210 THE PHOTO-MICROGRAPHY OF 



This blue medium practically stops all except the violet rays, 

 and leaves only the chemical focus. If now the image is as well 

 defined as before, the two foci are coincident ; if not, the distance 

 which the objective must be moved, to restore the definition, is 

 equal to the difference. In low powers this is apt to be marked. 

 With good modern objectives, the higher powers do not generally 

 present any difference. 



Often a dark edge appears, or the field is lighter in one part 

 than another. This is due to the heliostat being out of proper 

 relation to the condenser, so that it is not uniformly illuminated. 

 Unless this is corrected, the negative will not be of uniform den- 

 sity. When all is satisfactory upon the ground glass, a sensitive 

 plate may be substituted and the exposure made by intercei)ting 

 the light between the condenser and the stage with a bit of black- 

 ened cardboard, while the slide is being drawn out ; then lifting 

 the cardboard for the necessary time, then replacing it while the 

 slide is being drawn back. If the exposure should occupy several 

 minutes, the cardboard will not be necessary, since the time taken 

 up in pulling out and returning the slide is comparatively so short 

 as to be unimportant. 



The table upon which the apparatus stands should be very 

 firm, as any disturbance during exposure shows by blurred lines. 



The photographic treatment is a little different from that of 

 usual objects. The chief aim in photo-micrography is to get defi- 

 nition. The negative must be sufificiently dense to give a strong 

 print, in which the lights are high and the shadows deep, clear, 

 and sharply outlined. To this end the exposure must be short^ 

 barely long enough to get the details and yet to keep the shadows 

 clear — and the development excessive. This does away with the 

 soft middle tones and gradations between the lights and shadows 

 so esstintial to the beauty of landscajjc or jiortrait photography, 

 but so disastrous to micro-[)hotographs. 



Dry plates are useful with kerosene light ; the length of expo- 

 sure varies greatly, and must be ascertained for each particular 

 case. ^Vitli sunlight, low powers generally recjuire two blue cells 

 a quarter of an inch thick, and an ex[)osure as short as it can be 

 made by hand. If tlie specimen is of rather a deep-red colour 

 and covers the field pretty thoroughly, one cell may be sufftcient, 

 or none at all ma)' be necessary for tempering the light. 



