6 MICROSCOPY FOR BEGINNERS. 



the least touch it moves, the light reflected from the 

 mirror is lost, and the object is consequently left in semi- 

 obscurity. It is intended chiefly for the dissection of 

 flowers, grasses, or large insects, and fairly answers the 

 purpose if the observer desires to have both hands free, 

 and cares to screw the box to the table. But it is no 

 better than a good pocket-lens, which, with very little 

 trouble, can be attached to an upright rod and be used 

 for dissections ; in some respects it is much less valuable. 

 The three lenses supplied can be used as a single one or 

 combined. The former is good, the combination of two 

 is not seriously objectionable, but the focus of the united 

 three is, to the writer's eye, only five-sixteenths of an 

 inch, a distance, aside from the small field of view, that 

 effectually prevents its use as a dissecting microscope. 

 "With the lowest-power lens six letters of the type used 

 in this book can be seen, the focal distance being one 

 and one-fourth inches ; with two lenses combined, four 

 letters, with a focal length of about one -quarter of an 

 inch ; and with the three glasses only one letter is vis- 

 ible, the focal distance being five-sixteenths of an inch, 

 when tested by the writer. 



A " watch-maker's glass," winch is sometimes seen on 

 the microscopist's table, is a simple lens mounted in a 

 short horn or rubber tube, so arranged that it can be 

 held to the eye by the contraction of the muscles of 

 the cheek and brow, while both hands are used for 

 the manipulation of the object. It can be obtained 

 of various powers and focal lengths, but it is scarcely 



