of light through glasses of different
colors.

If the finger be placed in cold or warm water the temperature appears to
rise when a red glass is held in front of the eyes.

Experiment 174. _Formation of an inverted image on the retina_.
Take a freshly removed ox-eye; dissect the sclerotic from that part of
its posterior segment near the optic nerve. Roll up a piece of blackened
paper in the form of a tube, black surface innermost, and place the eye
in it with the cornea directed forward. Look at an object--_e.g._, a
candle-flame--and observe the inverted image of the flame shining
through the retina and choroid, and notice how the image moves when the
candle is moved.

Experiment 175. Focus a candle-flame or other object on the
ground-glass plate of an ordinary photographic camera, and observe the
small inverted image.

Experiment 176. _To illustrate spherical aberration_. Make a
pin-hole in a blackened piece of cardboard; look at a light placed at a
greater distance than the normal distance of accommodation. One will see
a radiate figure with four to eight radii. The figures obtained from
opposite eyes will probably differ in shape.

Experiment 177. Hold a thin wooden rod or pencil about a foot from
the eyes and look at a distant object. Note that the object appears
double. Close the right eye; the left image disappears, and _vice
versa_.

Experiment 178. _To show the movements of the iris_. It is an
extremely beautiful experiment, and one that can easily be made. Look
through a pin-hole in a card at a uniform white surface as the white
shade of an ordinary reading-lamp. With the right eye look through the
pin-hole, the left eye being closed. Note the size of the (slightly
dull) circular visual field. Open the left eye, the field becomes
brighter and smaller (contraction of pupil); close the left eye, after
an appreciable time, the field (now slightly dull) is seen gradually to
expand. One can thus see and observe the rate of movements of his own
iris.

[Illustration: Fig. 145.]

Experiment 179. _To show the blind spot_. The left eye being shut,
let the right eye be fixed upon the cross as in Fig. 145. When the book
is held at arm's length, both cross and round spot will be visible; but
if the book be brought to about 8 inches from the eye, the gaze being
kept steadily upon the cross, the round spot will at first disappear,
but a