282 Mr. Shavpe's Account of some Optical Experiments. 



This experiment proved the refraction caused by any edge 

 broiiglit near to a ray of light, more clearly and satisfactorily 

 than the former, but did not show whether the ray was re- 

 fracted from or towards the intercepting edge, as it was not evi- 

 dent on which side of the cobweb the ray passed. 



Expcr. 3. — I then looked at the candle through a slit be- 

 tween two parallel cards, sometimes ^^'^jth of an inch apart and 

 sometimes less, when the candle appeared, as in fig. 2, very 

 broad, with its edges red; nearer the middle it was yellow, and 

 the middle slightly blue or violet: the red and yellow much in 

 excess, because of the colour of the candle : there was also a 

 row of smaller candles on each side, each of which had its 

 outer edge red, and its inner one violet. 



When the cards were farther apart, the flame was less dis- 

 torted, as in fig. 3, with more but narrower traces of side 

 flames : when less apart, more distorted, as in fig. 4, with 

 fewer and wider side flames. 



In each case the width of the whole luminous appearance 

 was the same, because the refraction was the same ; but the 

 greater the number of rays which passed through the open- 

 ing, the greater the number of candles into which the coloured 

 rays re-arranged themselves. 



Exper: 4. — When I looked at the candle through a round 

 hole in the card of about the same diameter (4:'^th of an inch), 

 the effect was much the same, but less regular in appearance, 

 from its being circular : but when I lessened the hole and came 

 within two feet of the candle (for in the former experiments 

 I had been about ten feet distant), the distinctions of colour 

 ■were lost, and the hole appeared as in fig. 5, with a distinct 

 luminous centre, but the outer part was marked with dark 

 rays diverging from that centre. 



When I closed the eye-lids a little, the centre remained the 

 same, but parts of the top and bottom of the outer circle were 

 darkened as in fig. 6: when the eyelids were more closed, still 

 more of the top and bottom was darkened, as in fig. 7, plainly 

 proving that light enters the eye through the iris as well as 

 through the pupil ; the luminous centre being made by those 

 rays which passed uninterruptedly through the pupil, and the 

 radiated outer circle being the shadow of the iris as thrown 

 on the back of the eye, unless lessened by the intervention of 

 the eyelid, as in figs. 6 and 7. 



The bright reflection of a window or candle on the bulb of 

 a thermometer is surrounded by the same radiations, from the 

 same cause. 



Exper. 5. — If a card be held so as just to conceal an object, 

 the card will appear fringed with the object, or the object may 



appear 



