76 DARK-GROUND ILLUMINATION WITH HIGH POWERS [Ch. II 



part of the aperture of the objective. In the first method the aperture 

 of the condenser must be great and that of the objective moderate, 

 while in this the reverse is the case, and the objective should have a 

 large aperture and the condenser a moderate aperture. The solid 

 cone of light used for illumination has some of its rays deflected by 

 objects in the field so that they enter the marginal zones of the objec- 

 tive. To secure dark-ground illumination in this manner only these 

 marginal rays are utilized for the image, and the central solid cone of 

 light entering the objective must be eliminated. This is accomplished 

 by placing a diaphragm or stop on the back lens of the objective of 

 just the right size to cut out the central solid cone and allow the mar- 

 ginal rays to pass on to form the image. This gives fairly good re- 

 sults with all powers. The same may also be accomplished, as shown 

 by Gordon, 1906, by using a stop in the eye-point or Ramsden circle 

 (§ 57, % 22-24). 



Collateral Reading for Chapter II 



Carpenter-Dallinger. — The Microscope and its Revelations. 



Chamot, E. M. — Elementary Chemical Microscopy. 



Spitta, E. J. — Microscopy, the Construction, Theory and Use of the Microscope. 



Wright, Sir A. E. — Principles of Microscopy. 



Conrad Beck's Cantor Lectures of the Royal Society of Arts, 1907. (Dark- 

 ground illumination.) 



Nelson, E. M. — The Substage Condenser, its History Construction and Manage- 

 ment; and its Effect theoretically considered. Jour. Roy. Micr. Soc.,vol. XI, 

 1 89 1, pp. 90-105. 



