120 THE FROG CHAP. 



scopes have a fine adjustment, usually consisting of a spring 

 concealed in the pillar, and acting upon the horizontal arm 

 which carries the tube : it is worked by a screw (k), and by 

 means of it the tube can be adjusted to within 10^00^ f 

 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 adjust- 

 ing the mirror until a beam of light from the window or lamp 

 is reflected vertically upwards : a small hole in the dia- 

 phragm 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 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 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 

 microscope, 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 J oz.) of cover-glasses, small pieces of 

 very thin glass, to be had at the optician's. The most 

 convenient size is j 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 fth 

 inch in diameter ; and one or two dipping tubes, 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. 



