26 SELECTION AND USE 



Essential Parts of tlie Microscope. When a good 

 lens is he id 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. 

 If merely held in the hand the corpuscles will probably be in- 

 visible. Hence the importance of providing efficient means for 

 adjusting the focus and holding and illuminating the object, 

 and the object of the stand is to furnish these means in a com- 

 pact and convenient form. Every microscope, therefore, 



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

 the text is uot precisely the focus of the lens itself, but the focus of a 

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

 tance varies with different eyos, and so do< 8 the apparent size of objects. 

 To short sighted people objects appear of larger size than th< y do to 

 persons of ordinary eye-sight. In working with the compound microscope, 

 we frequently find that different people require a different focal adjust- 

 ment. 



