THE MICROSCOPE AND ITS PARTS. II 



qualities in the image. This addition is occasionally 

 useful, but it is not necessary. If the reader should 

 select an instrument with a body of the standard 

 length, and find that it is without a draw-tube, he 

 need not be troubled. The stand will be as valuable 

 without as with this secondary part. 



The eye-piece consists of two lenses at the opposite 

 ends of a short brass tube divided internally by a 

 diaphragm. The lens nearest the observer's eye 

 when the instrument is in use is the "eye-glass," the 

 one at the opposite extremity the "field-glass." The 

 price of the stand usually includes one or more eye' 

 pieces. If but one is supplied, it will generally be 

 the lowest power, the two-inch or "A;" if two, the 

 one-and-one-half or the one-inch, often also called 

 "B" or "C", will be added. Opticians also make f, ^, 

 ^, and even ^ inch eye-pieces, most of which are for 

 special kinds of microscopical work, their magnifying 

 power being enormous and the result almost worthless; 

 indeed, these very high-power eye-pieces are usually 

 to be avoided. On no account should they be selected 

 by the beginner in microscopy. Every purchaser of a 

 stand should insist upon having the two-inch, if he 

 can have but one, as it is always useful, and is the 

 only one that he will need for a long time, or until he 

 desires to use an eye-piece micrometer for' the measure- 

 ment of microscopic objects, when he can add the 

 one-inch, or "B" ocular to his stand. 



The lower 'opening in the body always carries a 

 screw to receive the screw on the upper end of the 

 objective. Several years ago the size of these screws 

 varied widely in stands and objectives of different 

 makers, so that if the student desired an objective of 



