PLATE 41. 



Diatoms. 



A Actinoptychus heliopelta x 270 

 B PART OF THE SAME DIATOM x 1250 



A B 



Objective Leitz No. 3. ". Leitz T y Achromatic, 



oil immersion. 

 Ocular ... ... Watson's No. 2x6. 



Condenser ... Watson's Universal Achromatic. 



Illuminant ... Liliput 4 amp. arc. 



Aux. Condenser Double convex lens, giving parallel ligkt. 



Screen Green. Wratten's B. 



Plate Imperial Process. Wellington's Anti- 



Screen. 



Exposure 1 min. 10 sec. 



A gives the general appearance of the diatom, but does not exhibit 

 any secondary structure. The condenser was stopped down to give 

 depth of focus, as this diatom is comparatively thick. 



B The secondary structure dots are shown in the primary 

 hexagonal areolations. These can be easily made out with an 

 immersion objective of T y or y y focus, and no difficulty should be 

 experienced in demonstrating them with a good |" or tr". 



C Pleurosigma angulatum x 720 



Baker *._.* Achromatic oil immersion objective. Leitz No. 4 

 ocular. Abbe condenser. Liliput arc lamp. Bull's-eye 

 condenser. Green screen, Wratten's B. Imperial process 

 plate. Exposure 40 sec. 



This photomicrograph was taken with the simple box camera 

 described on page 21 and Fig. 9, to show that with care satisfactory 

 results can be obtained with the simplest apparatus. 



D Navicula lyra x 1035 



Leitz 2mm. Apochromatic, oil immersion, N.A. 1.32. Zeiss 

 compensating ocular x 4. Watson's Universal Achromatic 

 condenser N.A. 1.0. Incandescent gas. No auxiliary con- 

 denser. No screen. Imperial process plate. Exposure, 

 10 min. 



This photomicrograph shows the "white dot resolution"; by a 

 slight alteration in the fine adjustment the dots appear black. The 

 white dots provide a good test for colour correction. There is a slight 

 colour with achromats, but the dots must be brilliant and sharply 

 defined. 



Photos. W. B. R. To follow Plate 40 and page 256. 



