22 SELECTION AND USB 



Essential Parts of the Microscope. When a good 

 lens is held steadily at a certain distance from an object which 

 is properly illuminated, this distance depending upon the form 

 and material of the lens, we are enabled to see the object clearly 

 and distinctly. When, however, this distance is either in- 

 creased or diminished, the object becomes blurred and indis- 

 tinct. The point at which vision is most distinct is called the 

 focus* of the lens, and when we are able to see it clearly the 

 object is said to be in focus; when the distance is either in- 

 creased or diminished, it is said to be out of focus. An object 

 is said to be within the focus when the lens is too near it, and 

 beyond the focus when the lens is too far away. 



The performance of any lens depends greatly upon the ac- 

 curacy with which it is adjusted to the correct focal distance, 

 and the steadiness with which it is held there. For all ordi- 

 nary purposes, lenses which do not magnify more than ten 

 diameters may be very conveniently held in the hand without 

 any special means of support; but when the power is much 

 greater than this, or where, as in the compound instrument, 

 the microscope is bulky and heavy, it becomes necessary to use 

 some mechanical contrivance which will hold the microscope 

 steadily in its position in relation to the object, otherwise the 

 view becomes indistinct. Thus a good lens, magnifying from 

 thirty to forty diameters, will very readily show the individual 

 corpuscles or globules in the blood of the frog, provided it is 

 arranged on a steady support and accurately adjusted for focus. 

 But if the lens be merely held in the hand, without any firm 

 support, it will be impossible for the observer to see the 

 corpuscles. 



Another important point is the illumination of the object. 

 Unless the object be properly illuminated it cannot be dis- 



* It is scarcely necessary to inform the reader that the focus described 

 in the text is not precisely the focus of the lens itself, but the focus of a 

 compound lens of -which the eye forms one element. Hence the focal 

 distance varies with different eyes, and so does the apparent size of ob- 

 jects. To short-sighted people objects appear of larger size than they do 

 to persons of ordinary eye-sight. In working with the compound micro- 

 scope we frequently find that different people require a different focal 

 adjustment, 



