36 ELEMENTARY CHEMICAL MICROSCOPY 



the image sways and the bright spot of light is displaced to one 

 side of the center the Abbe condenser is faulty and the character 

 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 centering screws, which permit moving the combina- 

 tion of lenses so that the optic axis of the condenser lens 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 will in fact be found a 

 distinct gain in ordinary practice in the illumination 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. 



When the recognition of colors becomes important, as, for 

 example, in microchemical analysis, the student must remember 

 that the image obtained in the microscope by illumination, 

 by mirror and Abbe condenser with light from the sky will 

 almost never show the true colors present in the object. To 

 obtain a properly colored image, slide a piece of pure white 



