CAllE OF THE EYES. 13 



One more ociilistic 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 any- 

 thing. 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 teiins with his eyes. 



