TESTING THE MICROSCOPE ITS USES. 7 



The first named is usually fitted with a f and a \ inch lens ; 

 in addition to these a T V immersion may be used for the triple 

 nose-piece. 



How to Tceep the instrument clean. After using the instru- 

 ment it should always be wiped dry, as it is damp from the 

 moisture of the breath and hands. The lenses should be re- 

 turned to their cases, and, if necessary, the surfaces are to be 

 rubbed off with a bit of soft chamois skin or fine linen. Water 

 will remove almost all the dirt from the anterior lens, but occa- 

 sionally it may be necessary to use alcohol. In such cases but 

 very little is requisite, as it may penetrate behind the anterior 

 lens and dissolve the Canada balsam that cements the different 

 portions together. 



It is well for the student to familiarize himself at first with 

 certain common objects that are apt to be met with in all 

 forms of microscopic work, such as the little foreign substances 

 that go to make up the dust of rooms ; these include minute 

 bits of wood, cotton and linen fibres, particles of wool, hairs of 

 various animals, feathers, etc. 



The imperfections in the glass should also be noted, and 

 especially the curious red figures sometimes resembling butter- 

 fly wings, caused by an accumulation in the flaws of the glass 

 of a red substance the red oxide of iron used by manufac- 

 turers in polishing glass. These red figures are often wonder- 

 fully alike, and have given rise to singular errors among micro- 

 scopical workers. 



TESTING THE MICROSCOPE ITS USES. 



Magnifying power of a lens. To determine the actual 

 magnifying power of a lens in combination with the particular 

 eye-piece that happens to be in use, the ordinary method is as 

 follows : 



The glass stage micrometer, which is ruled off into tenths, 

 hundredths, and thousandths of an inch, is placed upon the 

 stage and focussed. This having been done, the wooden rule, 

 which we have already alluded to and which is divided into 

 inches and tenths of an inch, is laid alongside of the micro- 

 meter-slide. 



One eye, looking outside of the tube, reads off the number 



