ON INSTRUMENTS AND THEIR USE. 31 



glasses made by Zeiss, Reichart and others, can 

 scarcely be surpassed. 



Flatness of field, penetration, definition, and 

 entire absence of colour are the essentials of a 

 thoroughly good objective, and all objectives not 

 possessing these qualities should be rejected. 



For working powers objectives with the lower 

 angles of aperture are to be preferred, as it is most 

 desirable to secure as much room as possible for 

 manipulation between the preparation and the 

 objective. For subsequent examination and study, 

 however, high power objectives with high angles 

 of aperture are invaluable, and oil-immersion objec- 

 tives especially so. A thoroughly good fine adjust- 

 ment is all-important to a good working microscope. 

 The stage of a microscope intended for work should 

 be a simple table, either circular or square (pre- 

 ferably the former), so that the specimen may be 

 rotated and all its parts easily got at ; there should 

 be no mechanical arrangements whatever upon it 

 (unless these are removable), so that the hands 

 may have free scope ; and the stage should be of 

 ample size, so that if necessary it may form a rest 

 for the hands whilst working. 



For exhaustive observations and delicate investi- 

 gations and the study of permanent preparations, 

 the elaborate microscopes and beautiful apparatus 

 now obtainable are of the highest possible value 

 and utility, but for ordinary work they are not only 

 quite unnecessary but too complicated to admit of 

 satisfactory work being done under them with any 

 comfort or convenience, whilst their great costliness 

 and delicacy, and their liability to injury from 

 chemicals and anything like rough usage, render 

 them totally unsuitable as " working " microscopes. 



