62 



LIGHTING AND FOCUSING 



[CH. II 



using the microscope at first is due partly at least to the constant effort 

 on the part of the observer to remedy the defects of focusing the 

 microscope by accommodation of the eyes. This should be avoided 

 and the fine adjustment of the microscope used instead of the muscles 

 of accommodation. With a microscope of the best qualit3 T , and suita- 

 ble light — that is light which is steady and not so bright as to dazzle 

 the eyes nor so dim as to strain them in determining details— micro- 

 scopic work should improve rather than injure the sight. 



Fig. 60. Double Eye Shade. This is 

 readily made by taking some thick bris- 

 tol board 7 x 14 centimeters and making 

 an oblong opening with rounded ends 

 (0-0) and of such a diameter that it goes 

 readily over the tube of the microscope. 

 This is the?i covered on both sides with 

 velveteen and a central slit (5) made in 

 the cloth. This admits the tube of the 

 microscope and holds the screen in posi- 

 tion. It may readily be pulled from side to side and thus serves for either eye, or 

 for the use of the eyes alternately . 



7 x 44 



cm. 





,'o 



' s 



\ 



^ > 



Fig. 60 a. Adjusting Eye-Shade. This is prepared like the preceding by cov- 

 ering a card about 6 x 12 centimeters with black velveteen. A copper wire about 

 ? mm.(yi in. ) and of the right length is curved as shown in the figure. Its ends are 

 rounded, and finally it is put tinder the cloth and seived 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. 



% 117. Position and Character of the Work-Table. — The 

 work-table should be very firm and large (60 x 120 cm.; 24x48 in.), 

 so that the necessary apparatus and material for work may not be too 

 crowded. The table should also be of the right height to make work 

 by it comfortable. An adjustable stool, something like a piano stool is 

 convenient, then one ma}- vary the height corresponding to the neces- 



