CH. II] LIGHTING AND FOCUSING 61 



If the cloudiness cannot be removed as directed above, moisten one 

 corner of the cloth or paper with 95 per cent, alcohol, wipe the glass first 

 with this, then with' the dry cloth or the paper. 



Water may be removed with soft cloth or the paper. 



Glycerin may be removed with cloth or paper saturated with dis- 

 tilled water ; remove the water as above. 



Blood or other albuminous material may be removed while fresh 

 with a moist cloth or paper, the same as glycerin. If the material 

 has dried on the glass, it may be removed more readily by adding a 

 small quantity of ammonia to the water in which the cloth is moistened, 

 (water 100 cc, ammonia 1 cc). 



Canada Balsam, damar, paraffin, or any oily substance may be re- 

 moved with a cloth or paper wet with chloroform, benzin or xylene. 

 The application of these liquids and their removal with a soft dry cloth 

 or paper should be as rapid as possible, so that none of the liquid will 

 have time to soften the setting of the lenses. 



Shellac Cevient may be removed by the paper or a cloth moistened 

 in 95 per cent, alcohol. 



Brunswick Black, Gold Size, and all other substances soluble in 

 chloroform, etc., maybe removed as directed for balsam and damar. 



In general, use a solvent of the substance on the glass and wipe it 

 off quickly with a fresh piece of the lens paper. 



It frequently happens that the upper surface of the back combina- 

 tion of the objective becomes dusty. This may be removed in part by 

 a brush, but more satisfactorily by using a piece of the soft paper loosely 

 twisted. When most of the dust is removed some of the paper may be 

 put over the end of a pine stick (like a match stick) and the glass sur- 

 face carefully wiped. 



CARE OF THE EYES 



§ 116. Keep both eyes open, using the eye-screen if necessary (Figs. 

 60, 60a); and divide the labor between the two eyes, i. e., use one eye 

 for observing the image awhile and then the other. In the begin- 

 ning it is not advisable to look into the microscope continuously for 

 more than half an hour at a time. One never should work with the 

 microscope after the eyes feel fatigued. After one becomes accustomed 

 to microscopic observation he can work for several hours with the 

 microscope without fatiguing the eyes. This is due to the fact that 

 the eyes become inured to labor like the other organs of the body by 

 judicious exercise. It is also due to the fact that but very slight ac- 

 commodation is required of the eyes, the eyes remaining nearly in a 

 condition of rest as for distant objects. The fatigue incident upon 



