120 THE FROG CHAP. 



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

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

 within TD-oi><rth of an inch. 



When 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 viewed by transmitted 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 80, the other the high power about 300 to 

 400 diameters. One eye-piece is sufficient, and a sliding coarse ad- 

 justment 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 money 

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

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

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

 false economy. 



Requisites 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. They must be thoroughly cleaned 

 before being used. 



2. A supply (about 4 oz. ) of cover-glasses, small pieces of very thin 

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

 square. They are best cleaned by 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 between the finger and thumb, they 

 are very liable to be broken. 



3. One or two thin glass rods, about 6 inches long and ^th inch in 

 diameter ; and one or two dipping ttibes, or pieces of glass tubing about 

 6 inches long and -^-th inch in diameter. The ends both of rods and 

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



4. Three or four dissecting needles, made by sticking a fine sewing 



