$2 Scientific Proceedings, Royal Dublin Society. 
to mere points. The Entoptiscope not only affords a most conve- 
nient way of exhibiting this magnification of a microscopic object 
without the aid of a microscope, but also enables the observer to 
make an accurate tracing of the magnified image of the object on 
the ground-glass stage. Ifthe object be placed below the'pin-hole, 
it does not appear inverted; if above, it does; see Part I., §5, p. 53: 
the nearer the pin-hole aperture is to the eye, and the nearer the 
object is to the pin-hole, the greater the magnification. A brilliant 
source of light, the smallest aperture in the diaphragm, and the 
shortest eye-cups should be used. More light is obtained by using 
the clear-glass instead of the ground-glass stage. The micro- 
scopic object, liquid or solid, can be placed in the centre of the 
glass diaphragm of one of the eye-cups referred to in the next 
experiment (10). 
Mr. E. M. Nelson, Past-President of the Royal Microscopical 
Society, informs me, as this paper is going to press, that some 
years ago he exhibited to that Society the hexagonal structure of 
a diatom, “ Triceratinum,”’ by means of its shadow on the retina, 
obtained by pin-hole illumination ; the hexagons measured about 
seoath of an inch (0°01 mm.). It was found necessary to use a 
diminished image of a pin-hole in a card, obtained at the conjugate 
focus of a wide-angle lens of about half-an-inch focus. The 
hexagonal structure of the eye of a fly, g{oth inch, Mr. Nelson 
found quite easy to demonstrate to an observer in this way. 
(10). As the refractive index of the cornea is nearly the same, 
and that of the aqueous humour the same, as water, by immersing 
the eyes in water, refraction by the crystalline alone becomes 
effective. This method was employed by Dr. Thomas Young to 
demonstrate that accommodation was effected by a change in 
curvature of the crystalline’ The experiment can easily be made 
with the Entoptiscope. The pillar of the instrument is clamped 
vertically, and an eye-cup, fitted with a glass diaphragm below, 
is placed in position. ‘The cup is nearly filled with (not quite 
cold) water, and the eye immersed, so that the cornea is in contact 
with the water. Open the largest aperture (say 2 or 3 mm. 
diameter) : note (a) the focal length of the convex glass lens 
1 On the Mechanism of the Eye. Phil. Trans. for 1801, p. 23. 
