CH. II] LIGHTING AND FOCUSING. 59 



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 micro- 

 scope after the eyes feel fatigued. 

 After one becomes accustomed to mi- 

 croscopic observation he can work for 

 Fig. 59.— Ward's Eye-Shade. several hours with the microscope 

 without fatiguing the eyes. This is due to the fact that the eyes be- 

 come inured to labor like the other organs of the body by judicious ex- 

 ercise. It is also due to the fact that but very slight accommodation is 

 required of the eyes, the eyes remaining nearly in a condition of rest as 

 for distant objects. The fatigue incident upon using the microscope at 

 first is due partly at least to the constant effort on the part of the ob- 

 server to remedy the defects of focusing of the microscope by accommo- 

 dation 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 quality, and suitable 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 — microscopic work should improve rather 

 than injure the sight. 



Fig. 6j. Double Eye-Shade. This is 

 readily made by taking some thick bris- 

 tol board 7 x ij. centimeters and making 

 an oblong opening with rounded ends 

 (o — o) and of such a diameter that it goes 

 readily over the tube of the microscope. 

 This is then covered on both sides with 

 velveteen and a central slit (s) 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. 



§ no. Position and Character of the Work-Table. — The work- 

 table should be very firm and large (122 x 72 cm. ; 28 x 48 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 com- 

 fortable. An adjustable stool, something like a piano stool is conven- 

 ient, then one may vary the height corresponding to the necessities of 

 special cases. It is a great advantage to sit facing the window if day- 

 light is used, then the hands do not constantly interfere with the illu- 

 mination. To avoid the discomfort of facing the light a screen like that 

 shown in Fig. 58 is very useful (see also under lighting, § 58). 



