SOME PKOGKESSIVE EXAMPLES 801 



achromatism rather than for any other quality, as the N.A. 

 required to bring out the desired appearance is not high. 



It is essential that this object should be mounted dry, and 

 that the surface to be photographed should be in optical 

 contact with the cover-glass. Such preparations are at the 

 present time scarce, and good ones appear to be becoming 

 ever more difficult to obtain. 



PLATE VIII, FIG. 3. The Podura Scale (Lepidocyrtus 

 curvicollis). X 2500. 



Enlargement made from Fig. 2. 



The method adopted was to make a positive on a backed 

 Wratten & Wainwright lantern-plate, and from this to 

 make an enlarged negative, and to print direct from the latter. 



PLATE VIII, FIG. 4. Diatom : Pleurosigma angulatum. 

 X 1000. 



Objective, 6-mm. apochromat, by Carl Zeiss. (Tube-length, 

 250 mm.) 



Ocular, compensating, No. 8. 



Illuminant, electric arc, taking 10 amperes. 



Screen, filter blue-green. 



Plate, Imperial orthochromatic. 



Exposure, 20 seconds. 



Development, metol-hydrokinone. 



This difficult and much-discussed diatom may be photo- 

 graphed to show varying appearances. These appearances are 

 to a great extent dependent on the foe as at which the photo- 

 graph is taken, as by focussing up or down on the object either 

 a black- or white-dot image may be obtained. In the particular 

 portion of the frustule shown it will be seen that on one side 

 the black-dot image is seen, whereas on the other the white-dot 

 appears. In places some intermediate stages are shown. 



PLATE IX, FIG. 1. Diatom : Amphipleura pellucida. 

 X 1500. 



Preparation mounted in realgar. 



Objective, 2-mm. apochromat, N.A. 1-40 (corrected for 

 250-mm. tube-length), by Carl Zeiss. 



Ocular, No. 4 projection. 



Illuminant, electric arc, 10 amperes. 



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



