802 PKACTICAL PHOTO-MICKOGKAPHY 



Auxiliary condenser, Zeiss No. I. 



Screen, aqueous solution of methylene-blue with signal- 

 green glass to cut out the red otherwise transmitted. 



Plate, Imperial orthochromatic. 



Exposure, 4 minutes. 



Development with metol-hydrokinone. 



The illumination to bring out the lined structure requires 

 to be oblique. This may be obtained by putting a slotted stop 

 at the back of the sub-stage condenser in the manner already 

 described ; or by partially closing the iris -diaphragm and 

 racking it across the field (as may be done in Continental forms 

 of stands) so that only a small portion of the total aperture of 

 the condenser, and that at one side only, allows light to pass. 

 The light must pass through the condenser and impinge on to 

 the object in the direction of the length of the latter. The 

 exact amount of decentration of the sub-stage iris must be 

 determined by experiment and depends to some extent on the 

 character of the particular frustule in view. The largest 

 opening and the least amount of decentration possible should 

 be used, but the lines must be shown clear and bright. The 

 black lines should not exceed in width one-fourth of the width 

 of the intervening clear spaces. The result obtained depends 

 greatly on the character of the particular frustule chosen : 

 some easily show a good image, while others with any 

 arrangement of illumination will with difficulty yield a good 

 result. 



PLATE IX, FIGS. 2, 3, 4, and 5. Diatom : Navicula rhom- 

 boides. X 750. 



Objective, J-inch achromatic, by W. Watson & Sons. 



Ocular -projection, No. 2. 



Illuminant, quartz mercury-vapour lamp. 



Condenser, Leitz oil-immersion, N.A. 1-40. 



Screen, acid-green and filter-yellow-K, giving perfectly 

 monochromatic light, X 5461. 



The whole purpose of these four photographs is to show the 

 effect of varying the incidence of the illumination. A slotted 

 stop was placed at the back of the sub-stage condenser allowing 

 only a small portion of the latter to be filled with light. The 

 object was placed on a microscope with a rotating stage, and 



