26 HOW TO USE THE MICROSCOPE 



But if he is going to prosecute a serious course of 

 study, he must take into consideration the magni- 

 fication he is Hkelyto require in his particular line. 

 In general nature study I find myself using a 3-mch 

 objective most constantly, and a l|-inoh is next in 

 frequency of use ; in that particular line it is only 

 occasionally that I use a J-inch. In my geological 

 work I seldom use a higher power than l|-inch ; 

 while in botanical study I use my l|-inch, 1-inch, 

 and ^inch with much frequency. For bacterio- 

 logical investigation considerable power is needed ; 

 a |-inch is certainly required, and a y^'i^ich oil 

 immersion objective is often imperative. But bac- 

 teriology is highly speciaHzed work, and oil im- 

 mersion object glasses are for expert workers, for 

 whose benefit this simple httle work is not being 

 written. 



In brief, my own experience, which I beheve is 

 general, points to the most regular use of low powers, 

 and I think I am justified in advising the beginner 

 who can see his way to buying only one objective 

 with his stand to get a 1-inch. If he proposes to 

 buy two, let him get a 1-tnch and a J-inch ; and if he 

 is to have three, let him add to his ^-inch and 1-inch 

 a 2-inch or 3-mch glass. 



Eye-pieces, or oculars, are the next consideration, 

 and it has aheady been stated that the eye-piece 

 magnifies the image formed by the objective, and 

 its magnification depends upon power and tube- 

 length. The eye-piece must, of course, be thoroughly 

 good and able to magnify, but it must always be 

 remembered that its function is simply to magnify 

 and present to the eye an image. If through in- 



