It is because of Weber’s law that we are able to ignore the manifold changes of illumination to which we are exposed in the course of the daylight hours; that the painter, who cannot at all reproduce by his pigments the absolute intensities of light in nature, can nevertheless give us a recognisably true copy of any natural scene; and that a large block of seats in the concert-room, at a moderate distance from the stage, can all be sold at the same price and all have equal advantages for hearing. You will readily find other instances of its working, if you are clear as regards the principle involved; namely, that the less you have of anything, the less need be added, and the more you have, the more must be added, to make an appreciable difference; or, on the negative side, that you are not likely to notice any difference in your surroundings, so long as the relations of the stimuli remain unchanged. So Weber’s law furnishes yet another reason for the apparent stability of the landscape that we discussed on p. 63. Questions and Exercises (1) Mark out, by indelible ink, a sq. cm. upon the outer surface of the forearm. Make upon transparent paper three maps of the area, marking hairs, veins, etc. Work over the area (_a_) with the horsehair, for pressure spots; (_b_) with a warmed carpenter’s spike, for warm spots; and (_c_) with a cooled spike, for cold spots. Enter the spots, as you find them, on the maps; remember to dot the hair down for pressure, but to draw the spike slowly and evenly along the skin for temperature. Lay the three maps together, and note the distribution and the relative number of the spots. (2) After shampooing, the scalp is sensitive and irritable under the brush. Why? (3) When you are writing with a pencil, or prodding in a pool with a stick, the sensations seem to come from the end of the pencil or stick. What organs are involved? And why should the sensations be localised as they are? Try to think out some experimental means of attacking this question. (4) What sensations do you get in the act of yawning? What in that of swallowing? What unusual sensations do you have, from the face, after you have been running hard? (5) How do sour and sweet in the mouth affect the sense of touch? Make solutions, in varying strengths, of sugar and of the juice of some very sour fruit; leave plenty of time between observations. (6) Prepare some bits of apple, onion, and raw potato. Close your eyes and hold your nose; then pick up these morsels at random, and chew them. Can you tell the difference? How? (7) Is there any evidence of taste contrast? (8) Secure adaptation to the scent of camphor; breathe regularly, and note the length of time necessary for the odour to disappear. Now smell at vanilla, heliotrope, absolute alcohol. Do you smell them? Try to account for the result, arguing by analogy from what you know of colours.