Experiment 171.  Place a thin disk of cold lead, the size of a silver dollar, on the forehead of a person whose eyes are closed; remove the disk, and on the same spot place two warm disks of equal size.  The person will judge the latter to be about the same weight, or lighter, than the single cold disk.

Experiment 172.  Compare two similar wooden disks, and let the diameter of one be slightly greater than that of the other.  Heat the smaller one to over 120 degrees F., and it will be judged heavier than the larger cold one.

Experiment 173. To illustrate the influence of excitation of one sense organ on the other sense organs.  Small colored patches the shape and color of which are not distinctly visible may become so when a tuning-fork is kept vibrating near the ears.  In other individuals the visual impressions are diminished by the same process.

  On listening to the ticking of a watch, the ticking sounds feebler or
  louder on looking at a source 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.]