USE OF THE MICROSCOPE. 49 



seldom packed away in its case, as a certain necessary 

 stiffness of motion in its various parts might thereby be 

 lessened. Yet it should be kept free from dust and damp. 

 A bell-glass cover, or glass case, or a cabinet which will 

 admit the reception of the instrument in a form ready for 

 immediate use, is desirable. Before using, the condition 

 of objective and eye-piece should be examined as well as 

 of the mirror, and dust or dampness removed. Another 

 examination should be made before the microscope is put 

 away. 



Stains on the brass-work may be removed by a linen 

 rag, and dust on the mirror and lenses by a fine camel's- 

 hair brush, or very soft and clean chamois skin. Frequent 

 wiping will injure the polish of the lenses. 



The upper surfaces of the lenses in the eye-pieces and 

 the mirror will need the most frequent attention The 

 objectives, if carefully handled and kept in their boxes 

 when not in use, will seldom require cleaning. If the front 

 of the objective becomes accidentally wet with fluid it 

 should be at once removed, and, when reagents are used, 

 great care should be taken to prevent contact with the 

 front of the lens. 



Care of the Eyes. Continuous observation, especially by 

 lamplight, and with high powers, has doubtless a ten- 

 dency to injure the sight. To cease work as soon as 

 fatigue begins is, however, a simple but certain rule for 

 protection. This time will vary greatly, according to the 

 general tone and vigor of the observer. It is also impor- 

 tant to use the eyes alternately if a monocular instrument 

 is employed, as otherwise great difference both in the 

 focus and in the sensitiveness of the eyes will result. The 

 habit of keeping the unemployed eye open is a good one, 

 and, though troublesome at first, is not difficult to ac- 

 quire. It is well to protect the eye from all extraneous 

 light, and to exclude every part of the object except that 

 which is under immediate observation. The diaphragm 



4 



