6 THE STRUCTURE OF 



wLich the hands may be used for all purposes of 

 manipulation. 



A brief description, aided by the accompanying 

 illustration, will, it is hoped, suffice to make the 

 beginner acquainted with the various parts of this 

 important instrument. 



We have already mentioned that when powerful 

 lenses are used in the examination of small objects 

 the hand is not sufficiently steady to give a firm 

 support to the lens employed, and this is equally 

 true of the hand that holds the object. It is also 

 essentially requisite to have both hands free, for 

 the purpose of manipulation. Hence it becomes 

 necessary to devise some mechanical means for the 

 support of both the lens and the object. How 

 these wants have been supplied by the enterprising 

 skill and ingenuity of our opticians will be best 

 seen as we describe the various parts of which the 

 Compound Microscope consists. 



The most important part of the instrument is 

 undoubtedly that which carries the various lenses 

 or magnifying powers. These are contained in the 

 interior of the tube or body, A, which is usually 

 constructed of brass, and from 8 to 10 inches in 

 length. At the upper end of the tube is the eye- 

 piece, B, so named from its proximity to the eye of 

 the observer. It consists of two plano-convex 

 lenses, set in a short piece of tubing, with their 

 flat surfaces turned towards the eye, and at a 

 distance from each other of half their united focal 

 lengths. The first of these lenses is the eye-glass, 

 while that nearest the objective is termed the field 

 lens. The use of the latter is to alter the course 



preference to any. The general excellence of these instru- 

 ments is so well known and the names of their makers are 

 so universal that the student will find no difficulty in provid- 

 ing himself with an efficient instrument at a moderate cost. 



