THE MICROSCOPE. 7 



Lister, a gentleman engaged in business in London, 

 who, in a paper published in the Philosophical 

 Transactions for 1829, pointed out the way in 

 which the Compound Microscope could be con- 

 structed free from chromatic and spherical aberra- 

 tion. 



It is this instrument, then, which is most com- 

 monly employed at the present day, and to which 

 we are indebted for most of the recent progress in 

 microscopic observation. Its essential parts, in 

 addition to those we have seen in the Simple 

 Microscope, are the tube and the eye-piece. Its 

 advantage over the simple instrument not only 

 consists in its magnifying power, but also in the 

 facility with which it may be employed, in conse- 

 quence of the more favourable distance the eye of 

 the observer is from the object observed. 



In using the Microscope, a great variety of acces- 

 sory apparatus may be employed to facilitate the 

 various objects which the observer has in view. 

 As this is a book for beginners, we shall only men- 

 tion a few of these. 



Every Microscope is generally supplied with 

 small slips of glass, three inches long and one inch 

 wide. These are intended to place the objects on, 

 which are to be examined. They are either used 

 temporarily or permanently with this object in 

 view, and are called slides. When used tempo- 

 rarily, an object, such as a small insect o part of 

 an insect, is placed upon the middle of it ; and it 

 may be either placed immediately upon the stage 

 at the proper distance from the object-glass, or a 

 drop of water may be placed on the slide and a 

 . piece of thinner glass placed over the object. This 

 is the most convenient arrangement, as you may 

 then tilt your Microscope without the slide or 

 object falling off. 



