158 BACTERIOLOGY. 
: ‘What solution is used as a double stain? 
Water that has been colored deeply with a filtered saturated 
alcoholic solution of methylin blue. The cover-glass preparation, after 
having been decolorized and rinsed in water, is allowed to remain in 
this solution about one minute. The portions of the specimen that 
have been decolorized will appear colored blue, while the tubercle bacilli 
will show up bright red. 
THE MICROSCOPE. 
What is the ocular or eyepiece ? 
It is the lens at which the eye is placed in looking at an object. 
What is the objective ? 
It is the lens that comes nearest to the object to be examined. 
What is the stage ? 
It is the platform of the instrument upon which the specimen to be 
examined rests. 
What is the reflector ? 
It is the mirror placed beneath the stage and is used to throw the 
light upon the object to be examined. 
What is the diaphragm ? 
It is the aperture in the stage through which light is thrown from 
the reflector upon the object. 
What is the coarse adjustment? 
It is either a rack and pinion or a sliding tube arrangement, by 
which the barrel of the instrument may be raised or lowered quickly in 
getting the correct focal adjustment. 
What is the fine adjustment ? 
It is a fine micrometer screw that serves to raise or lower the barrel 
of the microscope very slowly and gradually in order to get the exact 
focus of the object. 
What is an oil immersion objective ? 
It is sometimes called a homogeneous objective, and is composed of a 
system of lenses so constructed that it can only be used when there is 
a drop of oil between it and the object. The oil used is a specially 
prepared cedar oil. A drop of this is placed on the cover glass over the 
specimen, and the lens is brought in contact with it. This drop is of 
the same index of refraction as the glass slide, and therefore no light 
