4 77/ A' USE OF THE MICROSCOPE 



dark field. Hence trainiiij^ in the use of dark-field conden- 

 sers of the highest aperture is essential for obtaining the 

 best images of submicroscopic or nearly submicroscopic 

 objects. 



Thus without the correct use of each of these aids: 

 fine motion, correction for cover-glass, condenser iris, 

 adjustment of corrected and immersed condenser, hght 

 screens or yellow-green hght filters, diaphragm on source of 

 light, water-immersion objective, and dark field, the 

 optimum of vision through the microscope cannot be 

 regularly attained. 



Use of the Microscope. — 1. When the rules are nearly 

 all unknown or disregarded, the microscope is put before 

 the window or lamp, and the mirror turned till the hght is 

 seen through the eyepiece. The shde is then put on the 

 stage and focused. If the hght is too bright, it is cut down 

 by the iris of the uncorrected condenser, or by lowering 

 the condenser. The concave mirror may also be tried 

 with the high powers, when the hght is too dim. The 

 cover-glass may vary from 0.08 to 0.24 milHmeter, or 

 more. 



2. When all, or nearly all, of the rules are regarded, 

 the procedure differs according as the object is in a watery 

 fluid (of refractive index about 1.33), or in immersion oil 

 or balsam (of refractive index about 1.52). Suppose one 

 has to examine (a) a smear preparation of the pachytene 

 chromosomes of a plant, mounted in iron-acetocarmine 

 (of refractive index slightly over 1.33); and (b) a similar 

 preparation stained with iron-brazilin and mounted in 

 immersion oil. In both cases, the slides will have been 

 chosen as 1 millimeter thick, and the covers as 0.16 to 0.17 

 millimeter, (a) The microscope is centered with the 

 lamp, and placed at the known right distance from it. 

 (6) A properly treated double-ground glass disc has been 

 placed close before the appropriate electric lamp, (c) A 

 chosen yellow-green screen is put close before the reflecting 

 prism, (d) The microscope is duly slanted, and the 

 reflecting prism turned to hght up the field of the 10 objec- 



