74 SELECTION AND USE 



A good two-third, one-fifth, and one-tenth, giving magnifying 

 powers of from 50 to 1000 diameters, will, in general, answer 

 most requirements. It must be borne in mind, however, that 

 beginners can hardly be expected to use a one-tenth inch objec- 

 tive to great advantage, and, therefore, the purchase of this 

 item may safely be deferred. 



For the study of botany, and the ordinary facts of vegetable 

 physiology, a power of 300 is sufficient; but the very minute 

 forms of vegetable life require a much higher power, and so 

 do certain of the higher points in the physiology of plants. 



For the detection of adulteration, Hassal recommends the 

 inch and the quarter-inch objectives, giving a magnifying power 

 with No. 1 and No. 2 eye-pieces, of from 60 to 350 diameters. 

 For ordinary purposes of instruction and amusement in the 

 household, a microscope magnifying from 30 to 150 diameters 

 wiD be found most satisfactory, and for these reasons: Such an 

 instrument is easily managed; if well made it gives a power 

 amply sufficient for all ordinary objects, and it need not be ex- 

 pensive. Moreover, while it is an easy matter to prepare ob- 

 jects so that they maybe seen satisfactorily under low and me- 

 dium powers, it requires great skill and long practice to enable 

 the student to prepare objects so that they may be examined 

 with profit under a high power. And finally, under a high power, 

 but a very small portion of any ordinary object can be seen at 

 once, and consequently many of those things that are best 

 suited for popular examination can only be seen piecemeal 

 a very unsatisfactory mode of proceeding. Thus, under a power 

 of 750 diameters, a fly's foot could not possibly be seen as a whole ; 

 we might examine a single claw or pad at a time, but not the 

 whole foot, and consequently would find great difficulty in ac- 

 quiring an idea of what the general structure of the foot is. 

 To give the reader clearer ideas upon this point, we have just 

 measured the diameters of the fields seen under French and 

 American objectives, with the following results: With a magni- 

 fying power of 25 diameters, the field is about a quarter of an 

 inch; with 50 diameters, it is one-eighth of an inch; with 100 

 diameters, one-sixteenth of an inch; with 500 diameters, one- 

 eightieth of an inch; and with 1000 diameters, the one-hundred- 

 and-fiftieth of an inch, a space which is ordinarily invisible 



