184 METHODS OF MICROSCOPICAL RESEARCH. 



working with liigli powers and a long camera. It 

 must work with perfect smoothness and without 

 loss of time in either direction. If the tube of the 

 microscope can be shortened by unscrewing the 

 part above the fine adjustment so much the better, 

 for when working without the eye-piece, a long 

 tube, especially if it be a narrow one, greatly 

 contracts the field. This is one objection to the 

 Jackson-Lister form of stand for photographic 

 work, and the difficulty here can only be got over 

 by selecting an instrument with a wide tube. There 

 is another objection to the retention of a long tube, 

 whether the eye-piece be employed or not. It is 

 certain to give rise to a " flare " of light by reflec- 

 tion from its inner surface, and flare, whether 

 arising from this source or from the setting of the 

 object glass, or from the interior of the camera 

 itself, is absolutely fatal to the production of clean 

 pictures, and results in the diffusion of a uniform 

 light all over the plate, which impairs the purity of 

 the shadows and produces a general fog. One chief 

 seat of this internal inflexion is the fine adjustment 

 tube. No amount of dead blackening, or even 

 lining with black velvet will completely stop it. 

 The only thing to be done is to interpose along the 

 course of the tubes of the microscope, and in the 

 camera, and even in the object glass itself, if neces- 

 sary, a series of diaphragms. These may be cut 

 out of cards with gun punches, and glued to narrow 

 rings of cork to give them a grip of the tube. They 

 and the corks also must be painted dead black with 

 water colour (lamp black), and their number, 

 position and aperture so adjusted that a line drawn 

 from the centre of the object glass to the edge of 



