% PHOTOGRAPHIC INTRODUCTION 



spectrum make them especially suitable. Most 

 of the photographs of high magnification were 

 taken with a Powell and Lealand apochromatic 

 T V N.A. 1'43, specially made for the photography 

 of bacteria. It possesses a very flat field without 

 loss of central definition, but has an inconveniently 

 short working distance. All the other apochromatics 

 were made by Zeiss viz., -J- N.A. 1*4, -J-, ^, 1 inch, a 

 35 mm. lens, and a Planar of 50 mm. focus. 



Projection eyepieces were mostly employed, with 

 the necessary camera extension to obtain the re- 

 quired amplification, but at times the ordinary 

 compensating oculars for higher magnification were 

 used in their stead. It is not a little curious that 

 occasionally certain specimens were better rendered 

 by the T \- and a high eyepiece with short camera 

 extension than with the projecting ocular and 

 greater camera length. 



With respect to Substage Condensers, a dry 

 apochromatic (N.A. '95) by Powell and Lealand was 

 always used for powers over ^, and an achromatic 

 by Zeiss or one by Conrady for all the others, 

 excepting for the 35 mm and Planar lenses, which 

 performed best when a Nelson's quasi-achromatic 

 doublet was employed. 



A limelight mixed jet by Beard was the illumi- 



