OF THE MICBOSCOFE. 117 



tended to produce great magnifying power, a diameter of two 

 to two-and-a-half inches was found to be absolutely necessary 

 to prevent the glare arising from internal reflection. An inch 

 and a quarter is a good size for ordinary instruments. Since a 

 very long body is inconvenient when the microscope is used in 

 a vertical position, the best instruments are furnished with a 



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

 varied at pleasure. As explained iri a former paragraph (page 

 20), 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, which may even be made 

 of smooth paper. But it must be borne in mind that any change 

 in the distance between the eye-piece and the objective affects 

 the corrections of the latter, just as a change in the thickness 

 of the covering glass would do. Consequently, unless the ob- 

 jective has a considerable range of correction, it may be impos- 

 sible to get good results when a very long draw-tube is used. 

 On the other hand, the draw-tube may be used to good ad- 

 vantage as a means of correcting for covering glass when non- 

 adjusting objectives are used. We have a one-fifth objective 

 now before us, with which we can see clearly the lines on the 

 P. angidalum on the balsam Probe Platte when the draw-tube 

 is out, but when the tube is pushed in, the view is foggy and 

 indistinct. This is due to a disturbance of the corrections. 



The insides of all draw-tubes and bodies should be well 

 blackened. When bright or white the glare greatly injures the 



