AND PHYSIOLOGY. 



383 



erfully reflect light may be distinctly seen when not half the 

 size of the foregoing. Thus gold dust the TT V 5th part of an 

 inch in diameter is visible in common daylight. Lines may 

 be more readily seen than points. Opaque threads 4 9V oth of 

 an inch in diameter can be seen when held towards the light. 

 Attention also greatly helps to discern minute objects, or at 

 least to retain them in sight when once pointed out. Thus 

 we are often able to see a faint star in the sky or a ship in the 

 horizon after they have been pointed out to us, although they 

 were not seen before. "I myself,' 7 says Ehrenberg, " can 

 not see 2 Tooth of an inch, black or white, at twelve inches 

 distance ; but having found it at from four to five inches 

 distance, I can remove it to twelve inches and still see the 

 object plainly." 



703. Color Blindness.— Many people are afflicted with an 

 inability to distinguish certain colors of the solar spectrum. 

 This is called Color Blindness, or Daltonism. 



704. Formerly it was supposed that this affection was very 

 rare, but later researches show it to be quite common. Ac- 

 cording to experiments made by Dr. Wilson upon 1154 per- 

 sons in Edinburgh in 1852 and 1853, it was found that — 



1 in 55 confounds red with brown. 

 1 in 60 confounds brown with green. 

 1 in 45 confounds blue with green. 



Hence one in every 17.9 persons is color blind. 



THE SENSE OF VISION IN ANIMALS. 



705. Tapctiim— Pupil — Nictitating Membrane— Harde- 

 rian Gland. — The general anatomy of the eye of mammals 

 differs but slightly from that of man. Between the sclerotic 

 and choroid in some animals is found another membrane of a 



How is it with brilliant particles? How is the ready vision of lines compared with 

 points? How small a thread can be seen by the naked eye? What effect has the fa : 

 miliarity with an object ? 703. What is color blindness ? How often is this peculiarity 

 found ? What colors are most commonly confounded ? 



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