52 Scientific Proceedings, Royal Dublin Society. 
actual retina, the cross-section of the cone of rays from the 
points a and b being shown at g, h, and i, 7. These are the circles 
of diffusion corresponding to those points, and so with every other 
point of the object. If now a stenopaic screen, SS, be interposed, 
the luminous cone which enters the eye 1s restricted to the pencil of 
rays from the minute area of the pin-hole; and, consequently, all 
the sections of the cone up to its apex, «.¢., the area of the circles of 
diffusion, are proportionately diminished, so that where the retina 
intersects the cone of rays, this section has become practically a 
point ; the circles of diffusion (which are areas of confusion of the 
image) have vanished ; and a sharp image of a is formed at /, and 
of 6 at gy. Moreover, the magnitude of the retinal image is greater 
than before, coinciding, as will be seen, with the exterior boundary 
of the rays. 
ies 6), 
If the screen SS were removed, lines drawn from g and f, 
passing through the nodal point & of the eye, determine the 
direction in which the retinal image would be seen projected 
from the eye, and an inverted magnified image of the object 
a, 6, would therefore appear at G, H. Owing to the large visual 
angle subtended by an object held near the eye, a microscopic 
object, such as a transparent scale, divided into fifths of milli- 
metres, can be clearly seen when held close to the eye, and viewed 
through a very minute aperture in an opaque screen. The 
amount of light reaching the eye through a minute aperture is, of 
course, very small; but if the light be sufficiently good, this simple 
method affords an excellent way of seeing minute objects, or of 
reading well-illuminated small print, when a lens is not at hand, 
or spectacles are inadequate. 
If, instead of holding a small object on the far side of the 
pin-hole screen SS, it be held between the screen and the eye, a 
