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POPULAR SCIENCE MONTHLY 



duced by irregular refraction in the eye, originating in what are known 

 as the stellate figures. The figures seem to be construction lines, as 

 it were, in the crystalline lens, and develop during its growth. They 

 are permanent in form, when adult years are reached, and may be 



Fig. 8. Structure Lines in Crystalline Lens. 



seen with ease by the methods commonly explained in books upon 

 experimental psychology. 



If white rays may be seen around a white star on a dark back- 

 ground, then black rays must be visible around a black spot on a 

 white background. They may be easily seen by screening the greater 

 part of the pupil and allowing light from a black spot to pass through 



its margin. This is best done by a small circular 

 screen on the point of a needle. By slight per- 

 severance all the principal rays seen on a star 

 may be perceived on the black spot. These are 

 always present in the eye, but are not com- 

 monly perceived, because they are drowned out 

 in the lighter background, and habit compels 

 us to disregard them. Their importance in 

 astronomical work is at once evident when I state 

 that with the head in a definite position, I found it easier to see cer- 

 tain lines on the planet Mars and those easier lines coincided in direc- 

 tion with the two black rays represented in Fig. 9. It is evident that 

 observations made with the greatest possible care ought to show these 

 canals like marks, and if two of these rays be parallel, as may easily 

 happen in an astigmatic eye, some of the canals should appear double. 



Fig. 9. Rays on a 

 Black Spot obtained by 

 screening all the Pupil 

 except the margin of 

 the (left) Side. These 

 rays are the two long rays 

 on the left in Fig. 7. 



