44 Scientific Proceedings, Royal Dublin Society. 
short-focus convex lens, or by reflection from either surface of a 
silver spoon, or, still better, by a pin-hole in a piece of card or 
metal held close to the eye. With this very simple arrangement 
anyone can make a self-examination of any small opacities that 
may exist within the field of vision in either of his eyes. 
Looking at a lamp or a bright surface through a pinhole in a 
eard held close to the eye, the circle of light that is seen is not, 
however, the enlarged image of the pin-hole, but is the shadow cast 
by the circular aperture of the iris; it isin fact a magnified image 
of the pupil of the eye, and can be seen to vary in size with the 
degree of illumination, as the pupil contracts or expands, when a 
light is brought near to or removed from the eye. It is a sharp 
shadow, as the source of light from the pin-hole is practically a 
point, and hence any irregularity in the edge of the iris is clearly 
seen, as is any obscurity, however small, in the path of the rays. 
A pencil of rays emanating from some luminous point is called 
homocentric light, as the rays have the same centre of divergence ; 
the smaller the pin-hole aperture, or the point of light, the more 
perfectly homocentric is the luminous pencil, and the sharper and 
more detailed becomes the shadow of any small object.' By such 
means not only can minute opacities in the path of the rays be 
perceived, but objects which only slightly ditfer in transparency 
or in refractive power from the medium in which they are 
suspended, can be detected, if the vision of the observer be not 
seriously impaired. 
§ 2. 
This method of self-examination of obscurities which lie in the 
path of the rays within the eyeball is termed entoptic observation 
(évrée, within, and omzicvc); a term first given by Prof. Listing of 
Germany, who published an important paper on the subject sixty 
years ago.’ ‘Two years previously Sir David Brewster had drawn 
attention to the subject in a valuable investigation, wherein he 
1 The term Stenopeic, or Stenopaic (Gr. crevds, ‘narrow,’ omh, ‘ opening’) is usually 
employed to denote a screen with a small aperture. Stendpie would be a briefer and 
better term. If the botanists had not already used the word for another purpose, a 
micropyle (Gr. wixpds, ‘minute,’ mvAn, ‘ gate’), or micropylic screen, would have 
been more expressive. 
2 Listing, Beitrag zur physiologischen Optik. Gottingen, 1845. 
