72 



LIGHTING AND FOCUSING 



[ CH. II 



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

 bination 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 surfaces carefully wiped. 



CARB OF THE EYES 



§ 127. Keep both eyes open, using the eye-screen if necessary 

 {Fig. 67); and divide the labor between the two eyes, i. e. 

 use one eye for observing the image awhile and then the other. In 

 the beginning it is not advisable to look into the microscope con- 

 tinuously for more than half an hour at a time. One never should 

 ■work with the microscope after the eyes feel fatigued. After one 



Fig. 67. Adjusting Eye-Shade. This is prepared by covering a card 

 about 6 x 12 centimeters with black velveteen. A copper wire about 5 mm. 

 (Y% in.) and of the right length is curved as shown in the figure. Its 

 ends are rounded, and finally it is put under the cloth and sewed carefully all 

 around. The card and cloth are then cut as shown. The flexible wire makes 

 it possible to put the screen on the tube at any level. 



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 accomodation is required of the eyes, the eyes 

 remaining nearly in a condition of rest as for distant objects. The 



