On the Refraction of Light. 35 1 



Images transmitted to the retina through a puncture made by the 

 point of a fine needle in a piece of sheet lead, do not appear so dis- 

 tinct or luminous as when viewed by the naked eye ; but seem to be 

 enveloped in a hazy brownness, whose intensity is inversely as the 

 width of the aperture. The solar spectrum viewed through this 

 puncture loses much of its brilliancy, and the nice distinction be- 

 tween the colors can no longer be observed. Whether the small 

 aperture be angular or slightly elliptic, it will, when applied close to 

 the eye, invariably present the figure of a perfect geometrical circle. 

 If the aperture be an incision of a line in length, it will assume the 

 figure of an oblong, rounded at its two extremities. The size of 

 these circles does not depend entirely on their proximity to the eye, 

 but on the contraction and dilatation of the pupil. 



When a number of punctures are made in the lead, at the distance 

 of one sixteenth of an inch from one another, and gradually approach- 

 ed to the eye, the opaque interval between them diminishes as their 

 diameters increase, till at length it is obliterated, and the whole ap- 

 pears diaphanous and filled with perfect circles cutting one another. 

 But the most paradoxical appearance attending this experiment is, 

 that the circumferences of these cutting circles, which receive at 

 least doubly as much light as their centres, are distinctly dark, while 

 their centres are illuminated. This anomaly is somewhat analogous 

 to that discovered by Grimaldi, whilst studying the inflection of light : 

 viz. "that a body actually illuminated may become more dark by 

 adding a new light to that which it already receives." 



When a number of long parallel apertures, about half a line apart, 

 are cut witli the point of a penknife through a plate of lead, and ap- 

 plied close to the eye, no opaque interval will be found. A similar 

 phenomenon occurs when a small body, as a wire or needle, is grad- 

 ually approximated to the eye. Thus, if we close one eye and raise 

 a needle, by degrees, from the page of a book, towards the other, 

 the needle will appear as it approaches the eye, to lose imperceptibly 

 its density and opacity, till at last it \v\\\ become diaphanous, letters be- 

 ing visible through it, and nothing of it remaining but a diffused shadow 

 or penumbra. This illusory transparency maybe observed to occur 

 in cylinders of even' a hne in diameter. It is moreover to be re- 

 marked that the shadow (if so we may term it,) into which the body 

 appears to resolve itself, is imbued with the color of the body. If 

 we look at the page of a book through a thick wire painted black, 

 the letters within the shadow will seem dark and denigrated; but if 

 we look dn-ough a white wire at a black surface, it will appear whitish. 



