OF TKE MICROSCOPE. ' 83 



The Body. The only point connected with the body of the 

 microscope which requires consideration is its size, and this 

 must of necessity vary so much according to the purposes to 

 which the microscope is to be applied, that no rule can be 

 laid down. Pocket microscopes are of necessity small; micro- 

 scopes intended to give very great magnifying power must be 

 large. A standard size is seven to eight inches in length. The 

 diameter is not of very great importance in bodies of moder- 

 ate length, but Beale says that in his long tubes, intended to 

 produce great magnifying power, a diameter of two to two-and- 

 a-half inches was found to be absolutely necessary. An inch and 

 an eighth is a good size for ordinary instruments. Since a very 

 long body is inconvenient when the microscope is used in a ver- 

 tical position, the best instruments are furnished with a 



Draw Tube, whereby, the length of the body may be 

 varied at pleasure. As explained in a former paragraph, (page 

 15,) when the distance between the eye-piece and the object- 

 glass is increased, the magnifying power is increased also. 

 The draw tube, therefore, gives us the means of varying and 

 adjusting the magnifying power of the microscope, and this is 

 sometimes of great use. Thus, suppose it were required to 

 draw an object to a scale magnified exactly one hundred diame- 

 ters; it might be impossible to procure an eye-piece and an 

 objective that, with a fixed length of body, would give exactly 

 this amplification, but when we are able to vary the magnify- 

 ing power by changing the length of the body, it is easy to get 

 at it exactly. This, however, is but one of many advantages 

 afforded by the draw- tube. If the objective be good, and the 

 eye-piece not very high, an easy and very satisfactory way to 

 increase the magnifying power of the microscope is to lengthen 

 the body by means of an additional tube. Dr. Beale, who has 

 been a most successful worker with high magnifying powers, 

 tells us that in practice he has found this plan so much more 

 advantageous than the use of a deep eye-piece, that he never 

 uses the latter. In some cases he has extended the length of 

 the tube to two feet with good results. "We have frequently 

 adopted this method, and where brass tubing could not be had 

 we have used smooth writing paper, and a little paste with 

 very good effect. 



