184 rSE OF THE MICROSCOPE. 



can be accurately measured, and the object-glass corrected by 

 certain rules presently to be laid down. Wben any object 

 has been under examination, and it has become necessary 

 either to remove it from the stage for some alteration, or to 

 substitute another in its stead, it will be found the best plan, 

 if the object-glass be of short focus, to turn it back from the 

 object before removing it ; by this means aU contact between 

 it and the object will be prevented, and the risk of injury 

 avoided. After a few trials the custom of turning back the 

 object-glass by the coarse adjustment wiU become habitual. 



Various re-agents, such as the acids or alkalies, may be 

 applied to objects when they are being viewed, by touch- 

 ing the edges of the glass cover with a small glass rod or tube 

 that has been dipped into the required fluid. A small quan- 

 tity will gradually insinuate itself between the glasses, and its 

 effects can be easily watched. When a large quantity of a 

 corrosive fluid is required, or if the object in fluid be not 

 firmly secured between the two glasses, it is advisable to 

 place the microscope upright, that the danger of losing the 

 object, or of having the fluid escape about the stage, may be 

 avoided. The glasses between which objects are placed 

 should be perfectly free from grease ; if not so, when the water 

 or other fluid is added, it will not run equally between them; 

 and large air bubbles will be formed, aU of which should be 

 got rid of, especially if they are in the neighbourhood of the 

 object, as they act like so many lenses, and occasion a disturb- 

 ance in the rays of light that are passing either through or 

 about them. 



Small living insects may be viewed in the live box or the 

 animalcule cages by transmitted light, and those that are 

 large, such as flies, may even be held in the forceps, and the 

 circulation in their legs readily seen, no glass being placed 

 beneath them. 



When the highest powers are required, the light of day 

 or of a lamp may be greatly increased by the use of the achro- 

 matic condenser, described at page 113. In order to get the 

 most light, the lamp should be brought as near the instrument 

 as possible, and the achromatic condenser so adjusted that 



