6 AQUATIC MICROSCOPY FOR BEGINNERS. 



upright rod and be used' for dissections; in some re- 

 spects it is much less valuable. The three lenses sup- 

 plied 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 dis- 

 tance, aside from the small field of view, that effectu- 

 ally 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 visi- 

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

 when tested by the writer's myopic vision. 



A "watchmaker's glass," which is sometimes seen 

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

 in a short tube of horn or of rubber, 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 manipulating of the object. It can be ob- 

 tained of various powers and focal lengths, but it is 

 scarcely desirable. The prolonged contraction of the 

 facial muscles necessary to keep it in place is very 

 fatiguing, and the vapor continually evaporating 

 from the front of the eye, being confined within the 

 tube, is sure to condense on the lens and obscure the 

 object. Everything a watchmaker's glass will do, a 

 good pocket-lens will do better. 



A "Coddington lens" is admirable in many respects. 

 Its magnifying power is usually great, the image it 

 forms is excellent and the field of view good, but the 

 focus is, as a rule, unpleasantly short. This, apart 



