THE MICROSCOPE. 3 



are called doublets, and when three they are termed 

 triplets. The magnify ing-glasses which are made 

 to be held in the hand, frequently have two or 

 three lenses, by which their power may be increased 

 or decreased. Such instruments as these were the 

 first which were employed by microscopic observers; 

 and it is a proof of the essential nature of this 

 part of the Microscope, that many of the greatest 

 discoveries have been made with the Simple Mi- 

 ^croscope. 



The nearer the glass or lens is brought to an 

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

 its details will be observed. 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 optician's 

 for a few shillings, yet the hand is unsteady ; and 

 if these glasses 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 (see fig. 3), 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 slide of glass. This is called the 

 stage. (Fig. 1, 6^.) Sometimes rack-work is added to 

 this stage, by which the objects can be moved upon 

 it backwards and forwards, without being moved 

 by the hand. Such an arrangement as this i» 



B 2 



