IO THE MICROSCOPE. 



the greater distance is called the "field-glass." The 

 object-glasses consist of three lenses, one of which is 

 constructed to correct certain optical defects or 

 "aberrations." Each of these three sets of lenses is 

 itself compound ; and upon the excellence of the lenses 

 especially the merits of a good microscope depend. 

 They ought to define objects with great clearness ; 

 there must be no haziness about the outlines of the 

 images, and plenty of light must be secured. If you 

 notice any coloured rings encircling any object you 

 are inspecting, your object-glass must be discarded ; 

 it has not been corrected for this defect, which is 

 known as " chromatic aberration," and will prove of no 

 value to you. The microscopist will find two object- 

 glasses quite sufficient to begin with ; perhaps the inch, 

 which will magnify, with No. i eye-piece, 30 or 40 

 diameters, and the quarter of an inch, which, with 

 the same eye-piece, will give a power of about 200 

 diameters, will be found the most generally useful. 



I need hardly tell you that a microscope should be 

 perfectly steady, whether the body be inclined at any 

 angle or stand in a vertical position ; no vibration 

 should be communicated to the body when the ad- 

 justment screws are turned for the purpose of focus- 

 sing. Every microscope should be capable of being 

 used in three different positions vertical, inclined, 

 and horizontal. Nachet's microscopes formerly could 

 be used only with the body in a vertical position -one 

 which is very trying to the muscles of the neck of the 

 observer if he is working for some hours at a time ; 

 they are now made to assume the three positions. 

 Then, again, the stage is a very important part of the 

 instrument ; it should be three* inches long, by two 

 and a half broad. Nachet's instruments are too small 

 for working conveniently. Underneath the stage of 

 every microscope there should be a revolving circular 



