CARE OF THE EYES. 273 



4. PRESERVATION OF THE INSTRUMENT. 



In order to preserve the Microscope in perfect condition, the 

 lenses of the objective and of the eye-piece should be carefully 

 cleaned after being used. The front surface of the objective should 

 always be examined after use, for if solidifying liquids, &c., are 

 left upon it they are very likely to cause injury. The greatest care 

 is necessary in the employment of chemical reagents during obser- 

 vation, especially those which act upon the lead of the flint-glass. 

 Chemical processes which develope sulphuretted hydrogen, chlo- 

 rine, vapours of volatile acids and similar gases, ought never to be 

 used in the room devoted to the Microscope. Chloride of calcium 

 and nitric acid, fuming hydrochloric acid, &c., require cover-glasses 

 as large as possible to protect the objective even at a normal tem- 

 perature, and must not be placed in a heated state under the 

 Microscope. 



If, notwithstanding all care, a lens has been touched by the 

 reagent, it must be washed with distilled water, and then cleaned 

 in the usual way with a fine linen cloth softened by repeated 

 washing, or with soft leather. Some microscopists use a fresh-cut 

 surface of dry elder-pith ; the small particles which adhere to the 

 lens may be removed with a clean camel's-hair brush certainly 

 an efficient though somewhat complicated process. 



If the Microscope is used daily it may be covered with a glass 

 shade or a cloth to protect it from dust. Many observers are not 

 accustomed to give much attention to these matters ; they simply 

 put the objective in its box, and cover up the eye-piece, leaving 

 the instrument uncovered. In our experience, Microscopes which 

 have been treated thus for years, even with the objective screwed 

 on, have not been injured. It is, however, always better to err on 

 the side of caution. 



To prevent the formation of moisture on the eye-glass during 

 observation, either the eye-piece may be slightly warmed or the 

 whole instrument placed in a warm room before use. 



5. CARE OF THE EYES. 



Those who have moderately good eyes will soon be able to 

 devote several hours daily to the Microscope without injury to the 



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