SOME PKOGKESSIVE EXAMPLES 295 



pyro of about 20/ to ensure the utmost contrast. The 

 aim in developing should be to get an image as nearly as 

 possible black-and-white. It is necessary in photographing 

 this diatom to have an objective of sufficient N.A. to bring 

 out the dotted structure, but the illuminating-cone must even 

 then be reduced. It is important to observe carefully the 

 image while reducing the cone, to make sure that the dotted 

 structure is not lost and that no diffraction-effects begin to 

 show themselves around the dots. 



PLATE V, FIG. 6. Unstained Living Yeast-cells, x 500. 



Objective, 6-mm. apochromat, by C. Zeiss, corrected for 

 250-mm. tube. 



Projection-ocular, 4. 



Vertical camera of the Van Heurck type, with fixed extension. 



Newman & Guardia reflex camera placed as described 

 on p. 140, so that direct observation of the image could be 

 carried out up to the moment of exposure. 



Illuminant, electric arc, taking 10 amperes. 



Leitz oil-immersion sub-stage condenser. 



The illuminating-cone had to be reduced to approxi- 

 mately one-half of the total N.A. of the objective to obtain 

 as great a contrast as possible ; the object itself being trans- 

 parent and depending for its visibility on the difference of 

 refractive index between the cell body and the medium in 

 which the organisms lie. 



Screen, 1% solution of methylene-blue, 2 mm. thick. 



Plate, Imperial flashlight, ordinary. 



Exposure, one-twenty-fifth of a second by means of focal 

 plane shutter. 



Development by methol-hydrokinone, with an excess of 

 hydrokinone to obtain contrast. Such preparations should 

 be mounted not in hanging drop, but in as thin a film as possible 

 between an ordinary slide and cover-glass. The chief difficulty 

 lies in getting a field of organisms which all lie in the same plane. 



PLATE VI, FIG. 1. Actinomycosis bovis. Section of 

 Tongue of Cow. x 350. 



The organism stained Gram, and counter-stained eosin. 

 Objective, J-inch achromatic, by Charles Baker. 

 Ocular, none. 



