ILLUMINATION OF OBJECTS; ABBE CONDENSER 27 



and the amount of the fault will be indicated by the magnitude 

 of image displacement. 



In the better grades of Abbe condensers the mounting is fitted 

 with two centering screws, which permit moving the entire con- 

 denser so that the optic axis of the condenser lenses becomes 

 coincident with the optic axis of objective and ocular. 



The simplest method for easily centering adjustable Abbe 

 condensers is to have a cap made, fitting exactly over the top 

 lens of the condenser; at the exact center of this cap an exceed- 

 ingly tiny hole is drilled falling in the optic axis of the apparatus. 

 The microscope is focused upon this hole, illuminated by the 

 light transmitted by the condenser and the bright spot seen is 

 brought by means of the centering screws so that its center is 

 coincident with the center of the field. 



It is the rule to always use the plane mirror with the Abbe 

 condenser; but when the windows of a laboratory have small 

 panes or wide cross bars it is often impossible to properly illu- 

 minate an object with the plane mirror and Abbe condenser 

 without projecting an image of the window bars into the field. 

 Either the microscope must be moved very close to the window 

 or the concave mirror must be used; the latter plan necessi- 

 tates closing the iris diaphragm two-thirds or more and lowering 

 the condenser. In aggravated cases a disk of ground glass may 

 be placed below the condenser or in front of the mirror. The 

 use of a disk of thin, fine ground glass below the condenser will 

 in fact be found a distinct gain in ordinary practice in the illu- 

 mination of most objects. By its use softer, clearer and more 

 easily interpreted images will often be obtained and the true 

 colors of objects will be more easily recognized. 



The ring attached to the lower part of the condenser and 

 arranged to swing aside serves to carry disks of blue glass to be 

 employed when working with artificial light. By this means a 

 much less fatiguing illumination is obtained, and providing the 

 proper intensity of blue glass is at hand, white light giving 

 proper color values is secured. Blue glass should always be 

 placed below the condenser when working with yellow artificial 

 lights. Most manufacturers supply blue glass disks with all 



