I20 THE FROG chap. 



pillar, and acting upon the horizontal arm which carries the tube : it is 

 worked by a screw (/■), and b)- means of it the tube can be adjusted to 

 within ijjjyjth of an inch. 



\Mien the object is transparent — as in most cases with which we shall 

 have to deal — it is placed over the hole in the stage on a glass slide, 

 and illuminated from below by adjusting the mirror until a beam of light 

 from the window or lamp is reflected vertically upwards : a small hole 

 in the diaphragm should be used with the high power. The object is 

 thus said to be vie\\*ed by iraiisniitted light. In the case of opaque 

 substances the mirror is not used, and the object is illuminated by the 

 light falling upon it directly : it is then said to be viewed by reflected 

 light. 



A student's microscope should have two objectives, one — the low 

 power — magnifying about So, the other — \h^ high power — about 300 to 

 400 diameters. One eye-piece is quite sufticient, and a sliding coarse 

 adjustment is nearly as convenient as a rack and pinion, besides being 

 cheaper and less likely to get out of order. The mistake often made in 

 choosing a microscope is to get one of elaborate construction, the mone)' 

 going largely in brass-work. The proper thing is to get the simplest 

 form of stand consistent with perfect rigidity, fitted with the best 

 possiijle fine adjustment and lenses : to save on either of the latter is 

 false economy. 



Reauisites for Microscopic Work.— in addition to the micro- 

 scope, the following will be required before starting work : — 



1. A few slides or slips of glass, 3 inches long by i inch wide, which 

 can be obtained from an optician. The)' must be thoroughly cleaned 

 before being used. 



2. .\ supply (about | nz.) o{ eovcr-gla^ses, small pieces ... very thin 

 glass, to be had at the optician's. The most convenient size is % inch 

 square. They are best cleaned liy being soaked for a few minutes in 

 strong nitric acid, and then thoroughly washed under the tap, after 

 which they should be dried by being placed flat on a clean surface and 

 rubbed with a handkerchief : if held betw een the finger and thumb, they 

 are very liable to be broken. 



3. One or two thin ^Azi-j rods, about 6 inches long and Jth inch in 

 diameter : and one or two dipping-tubes, or pieces of glass tubing about 

 6 inches long and i\th inch in diameter. The ends both of rod* and 

 tubes should be rounded off in the flame of a blow-pipe. 



4. Half a dozen disseiting needles, made by sticking a fine sewing 



