Marvels of Pond-Life. 9 



cover the cardboard with black velvet, the commonest, 

 which is not glossy, answers best, and your screen is 

 made. Put the hole over the tube of the microscope, 

 and let the screen rest on the little ledge or rim which 

 forms an ornamental finish to most instruments. A 

 piece of cork may be gummed at the back of the screen, 

 so as to tilt it a little, and diminish its chance of 

 coming into contact with that important organ the 

 nose. This little contrivance adds to the clearness and 

 brilliancy of objects, and is a great accommodation to 

 the eyes. 



One more oculistic memorandum, and we have done 

 with this chapter. Do not stare at portions of objects 

 that are out of focus, and consequently indistinct, as 

 this injures the eyes more than anything. Remember 

 the proverb, '• None so deaf as those that won't hear," 

 which naturally suggests for a companion, " None so 

 blind as those that won^t see." It is often impossible 

 to get every object in the field in focus at one time ; — 

 look only at that which is in focus, and be blind to all 

 the rest. This is a habit easily acquired, and is one 

 for which our natural microscopes are exceedingly 

 grateful; and every judicious observer desires to keep 

 on the best terms with his eyes. 



