TJie Working Aperture. By Edward M. Nelson. 245 



some particularly fine diatom resolution, or minute structure in a 

 nucleus, or a barely visible ilagellum, how difficult it is to reproduce 

 the conditions so that on a subsequent occasion the same delicate 

 structure may be made visible to some fellow worker in that line of 

 research. Although they are using the same Microscope and object, 

 with the same objective, eye-piece, and substage condenser, with 

 the same tube-length and collar correction, and the same lamp and 

 light filter, yet the image may for a long time elude their grasp. 

 The reason for this is not far to seek ; for microscopists in yearly 

 increasing numbers have iris diaphragms instead of a wheel of dia- 

 phragms fitted to their substage condensers ; and as it is well known 

 that a minute difference in the position of the handle of the iris will 

 cause a considerable difference in the size of the disc of light at the 

 back of the objective, it comes to pass that one of the conditions, and 

 that a most important one in the visibility of minute detail, viz. that 

 of an identically similar working aperture, is not reproduced, and 

 hence the failure of the demonstration. 



If the working aperture be accurately measured in the manner 

 indicated above, and recorded in addition to the other necessary 

 conditions, there is little doubt that much time and labour will be 

 saved in the reproduction of any particular microscopical image. 



June 19th, 1901 



