6 PHOTOGRAPHIC INTRODUCTION 



closed for the purpose of preventing " flooding n of 

 light ; and even then, not usually more than one- 

 third of the diameter of the back lens of the objec- 

 tive when looking down the tube of the microscope 

 after removing the eyepiece. We have found that 

 an additional iris diaphragm placed above the objec- 

 tive, known to some microscopists as a " Davis 

 diaphragm," is of great service with quite low 

 powers — it reduces the N.A. of the objective a trifie r 

 it is true, but it frequently gives greater depth of 

 focus and sharpens up the whole picture. 



The negatives were all taken without exception 

 on Edwards's Isochromatic medium ^-plates and 

 developed with hydrokinone and soda. Although 

 the isochromatism of the emulsion is not all that 

 could be desired, yet its uniformity and fine grain 

 have always been so pronounced that we have never 

 felt inclined to use plates by other makers, although 

 we have tried most of them. 



Exposure is always a difficult matter ; but as a 

 rough guide we may state that, using the pot-green 

 glass to which we have previously referred, the 

 isomedium plate, magnification at 1000 diameters, 

 and a strong limelight mixed jet, the exposure 

 varied from f- to l^ minutes, or less if the auxiliary 

 lens was used. It is not easy without practice at all 



