84 Scientific Proceedings, Royal Dublin Society. 
PART III.—OTHER ENTOPTIC PHENOMENA. 
(1). When the point of light is at a considerable distance— 
a, stenopic screen 4 or 5 feet away, with a bright light behind, or 
the incandescent gas-mantle of a street lamp over 40 feet away, does 
very well—beautiful diffraction phenomena will be seen ; lines of 
light, with spectrum tints, are seen radiating in all directions from 
the common centre. This, sometimes called the ciliary corona, is 
due to the stellate fibrous structure of the crystalline lens ; and the 
radiations I find become very brilliant and conspicuous when the 
lens has the numerous small opacities which occur in the growth 
of cataract. A rainbow-coloured ring is often seen (though I have 
not noticed it) surrounding these radiations, probably due to the 
epithelial cells of the cornea or the fibres of the crystalline. A 
few grains of hycopodium dust, or other powder, scattered on a 
glass plate, and held between the eye and a distant point of light, 
give rise to exactly similar diffraction phenomena. 
(2). If the stenopic screen, or other brilliant point of light, 
be placed at different distances from the eye, any astigmatism in 
the eye can be readily seen, and its meridian accurately deter- 
mined by the appearances presented. Here, again, Dr. Thomas 
Young was the first to employ this method of observation; and in 
his paper of 1801—to which reference has already been made— 
diagrams are given by Young, showing the various appearances of 
a luminous point at different distances from the eye. ‘T'scherning, 
in his “ Physiological Optics” (English translation, pp. 138-144), 
gives a series of drawings of the forms presented by a luminous 
point at varying distances from the eye. 
I find that a narrow illuminated slit, capable of rotation through 
180°, is far better for judging the amount and the exact meridian 
of astigmatism. Such a slit when slowly rotated from a vertical 
to a horizontal axis on either side also forms an extremely delicate 
method of judging whether spectacles to correct astigmatism have 
been accurately made or not. If there be any inaccuracy in the 
meridian or the curvature of the glasses, the slit will, in some 
position, appear with a faint duplicate image or ghost upon 
rotation. 
