Experiment 18. Astigmatism (effect of unequal curvature of cornea or lens along certain lines). With end of crayon draw about twelve straight, even lines crossing at one point on the blackboard. Have the lines of equal distinctness. How many pupils report that the lines in certain directions are blurred? Inquire whether these pupils have frequent headaches from eye strain.
Experiment 19. Can Sound reach the Ear through the Bones?—Hold a watch between the lips and notice its ticking. Close the teeth down upon it and notice any change in the sound. Cover one, then both ears, and note the result.
Experiment 20. Test keenness of hearing by having pupils walk away from a ticking watch until it becomes inaudible. Test each ear. A "stop" watch is preferable.
Experiment 21. Advantage of Two Ears over One.—Have the class stand in a circle. Blindfold some one and place him in the middle of the circle. Let various pupils clap the hands as the teacher points to each. Can the blindfolded one point in the direction whence the sound comes? Stop one ear with a handkerchief and repeat. Result? Conclusion? From what two points in the circle does the sound fall upon both ears alike?
Experiment 22. The Cause of Nasal Tones.—Let a pupil go to the back of the room and read a paragraph, and hold his nose until partly through the reading. Or the teacher may read with his face and hand hidden by a large book. Let the other pupils raise their hands when they notice a change in the quality of the reader's voice. Does the experiment show that a "nasal" tone comes partly through the nose or through the mouth only? Does stoppage of the nostrils by catarrh cause a nasal tone?
Five Differences between Special and General Sensation.—First, the nerves of special sense all end in special organs at the surface; for instance, the touch corpuscles are for touch, the eye is for sight, etc. There are many nerves in the body that do not end in special organs; these nerves give what is called general sensation. A second difference is that general sensation tells of the condition of the interior of the body, while special sensations tell us of the condition of the surface of the body and of the outside world. Third, general sensations are not so exact as the reports of the special senses. One can locate a point on the skin that has been touched much more accurately than he can locate an internal pain. A fourth difference is that the meaning of each special sensation must be learned (usually in infancy); but the meaning of general sensations is inherited. This inherited knowledge of what general sensations mean is also called instinct. Fifth, the sympathetic nerves
