H 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. l"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., ^ N.A. 1*4, ^, ^, 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 ^ 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 jr, 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- 



