SOME PKOGKESSIVE EXAMPLES 291 



No ocular. 



Illuminant, one-ampere Nernst lamp. 



Zeiss auxiliary collecting-lenses, Nos. I and III. 



Watson achromatic condenser with top lens removed. 



Critical image of auxiliary iris-diaphragm projected into 

 the object -plane. 



Green screen, acetate of copper and bichromate of potash ; 

 the acetate of copper being in excess so that the screen is 

 visually of a bright green, transmitting no red light. 



Camera-extension, 3 ft. 6 in. 



Exposure, 10 seconds. 



Plate, Wellington orthochromatic process. 



Illuminating-cone to fill two-thirds of the back lens of the 

 objective. 



Development by pyro-soda, with pyro in excess, and 

 five drops of 10% bromide of potassium to each ounce of 

 developer. 



These objects present no particular difficulty, and may be 

 satisfactorily photographed with any medium-power achro- 

 matic lens, provided the colour-screen used completely absorbs 

 all light of short wave-length. In developing, it is important 

 to see that contrast is secured, as the print should show a pure 

 black-and-white effect. 



PLATE IV, FIG. 4. Surface Colony of Ringworm Organism. 



X 2. 



Objective, 6J-inch Zeiss double-prolar anastigmat, on 

 camera-front. 



Illuminant, lime-light. A parallel beam projected on to 

 surface of plate by ordinary bull's-eye, at an angle of 45 

 to the optical axis. 



Screen, green pot-glass. 



Plate, Edward's isochromatic, medium rapidity. 



Exposure, 10 seconds. 



Development, pyro-soda. 



PLATE IV, FIG. 5. Surface Colony of Bacillus anihracis. 

 X 20. 



Objective, Leitz 75-mm. micro-summar. 

 No ocular. 

 Illuminant, lime-light. 



u 2 



