PHOTOMICROGRAPHY OF DIATOMS 255 



field and penetration, and these could not be obtained 

 by using an objective of high N.A. 



A comparison between the use of low- and high-power 

 objectives for diatom photography is seen in the illustra- 

 tion of Triceratium (Plate 40). In Fig. A, the diatom 

 was photographed with a low-power objective and long 

 camera extension ; the whole of the diatom is sharply 

 rendered, but there is no trace of secondary structure ; 

 whilst in Fig. B, which is of identically the same magnifi- 

 cation, but taken with an objective of higher N.A., the 

 secondary markings are clearly visible, but the photo- 

 graph is sharp only in the centre ; that is, resolution has 

 been obtained at the expense of penetration. 



Actinoptychus heliopelta (Plate 41, Figs. A, B) offers 

 another example of the necessity of employing a low- 

 power objective to obtain a sharp photograph of an entire 

 diatom. This object is by no means flat, and it is necessary 

 to use a narrow cone of light from the substage condenser 

 to obtain sharpness. When photographed at higher 

 magnification and with an objective of high N.A. the 

 secondary structure can be resolved without difficulty. 



Navicula lyra (Plate 41, Fig. D) is a favourite diatom 

 for photography. The dots can be made out with a good 

 J" objective. They should appear perfectly white, and 

 do so if the objective is an apochromat, but with an 

 achromat they are generally tinged with colour, but by 

 using a green screen excellent photographs can be 

 obtained. 



Navicula crabro (Plate 41, Fig. D) presents no special 

 difficulty ; we have photographed this diatom for the 

 purpose of showing how easily diffraction effects are 

 produced by closing the iris diaphragm too much. Two 

 negatives were taken with Swift's J" objective of N.A. 

 0-88, in one case working with nearly full aperture, in the 

 other case with the iris closed down to greatly restrict 

 the working aperture of the objective. In the illustration 



