CHAPTER XIX 



TESTING THE MICROSCOPE 



Tests of the microscope are sometimes necessary ; because 

 if the lenses are not kept optically clean, and the condenser 

 adjusted so as to give an aperture not much below that of 

 the objective employed, the maximum useful magnification 

 cannot be attained. 



1. Testing the Light. — The electric lamp when used in 

 the daytime is often brighter towards evening, apparently 

 because of a temporary increase in voltage of the local 

 current. This increase must be compensated by the use 

 of a resistance, or a more absorbent screen than the one 

 employed before. The intensity of the fight should be 

 lowered by screens until it is at its optimum. The incan- 

 descent ribbon or concentrated filament should be kept 

 opposite the center of the ground glass, so that the brightest 

 spot is diaphragmed for the high powers. 



2. Testing the Ground Glass. — If ground glass is used, 

 it should pass no perceptible amount of direct light. It 

 can be tested, as already noted, by forming a strong image 

 of the lamp filament on a screen by a large condensing lens, 

 and interposing the ground glass to be tested not far in 

 front of the lens. No trace of an image of the incandescent 

 tungsten should be seen on the screen. To make ground 

 glass equal to new, rub it with a saturated solution of 

 magnesium sulphate, and dry polish this off, as already 

 stated. 



3. Testing the Yellow-green Light Filter. — This is best 

 made of a gelatin film cemented by balsam between two 

 plane glass plates. It should be compared with a similar 

 unused gelatin film occasionally, to see if any fading has 

 occurred. Wratten filters Nos. 66, 56, and 58 do not fade, 

 perceptibly in several years of daily employment, if pro- 

 lyl 



