THE MICROSCOPE AND ITS PARTS. 11 



the proper length to obtain the best results from the ob- 

 jectives. In such a stand, when the inner, or " draw," 

 tube is pushed down, the microscope will have the low- 

 est magnifying power obtainable with the eye-piece and 

 objective then in use ; when fully extended, the power 

 of the objective will be greatly increased, so that by 

 varying the length of the body by the use of the " draw- 

 tube," many different magnifying powers may be ob- 

 tained from one objective. In some cases this arrange- 

 ment may be useful ; it is at least not entirely objec- 

 tionable, neither is it very convenient. Stands with an 

 undivided body ten inches long the standard length 

 also often have a draw- tube by means of which the body 

 can be enormously lengthened and the magnifying pow- 

 er enormously increased, but usually with a loss of some 

 good qualities in the image. The addition is occasionally 

 useful, but it is not necessary. If the reader selects an 

 instrument with a body of the standard length, and he 

 finds 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 dia- 

 phragm. 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 

 2 



