THE MICROSCOPE. 6 



and attached to the Simple Microscope, they are 

 called doublets and triplets. The magnify ing-glasses 

 which are made to be held in the hand, frequently 

 have two or three glasses, by which the power of 

 the instrument may be increased or decreased. Such 

 instruments as these were the first which were em- 

 ployed by microscopic observers ; and it is a proof 

 of the essential nature of this part of the Micro- 

 scope, that many of our greatest discoveries have 

 been made with the Simple Microscope. 



The nearer the glass or lens is brought to an 

 object, so as to enable the eye to see, the more of 

 the object will be seen. So that when we use a 

 glass which enables us to see within one inch of an 

 object, we see much more than if we could bring it 

 within only an inch and a half or two inches. So 

 on, till we come to distances so small as the eighth, 

 sixteenth, or even twentieth of an inch. 



Although a great deal may be seen by a common 

 hand-glass, such as may be purchased at an opti- 

 cian's for 7s. 6d. or half a guinea, yet the hand is 

 unsteady, and if they were made with a very short 

 focus, it would be almost impossible to use them. 

 Besides, it is very desirable in examining objects, 

 to have both hands free. On these accounts the 

 glasses, which in such an arrangement are called 

 object-glasses^ are attached to a stand and placed in 

 an arm which moves up and down with rack-work. 

 In this way, the distance of the object from the 

 glass can be regulated with great nicety. Under- 

 neath the glass, and attached to the same stand, is a 

 little plate or framework, to hold objects, which 

 are placed on a piece of glass. This is called the 

 stage. Sometimes rack-work is added to this stage, 

 by which the objects can be moved upon it back- 

 wards and forwards, without being moved by the 

 hand. Such an arrangement as this is called a 

 B 2 



