PRACTICAL EXERCISES 



than the test. The green-blind will choose (with reds) greens, greys, 

 or browns which are brighter than the test. 



It must be remembered that the results of tests with the coloured 

 wools need not be precisely the same as those with coloured lights, 

 and that when there is a discrepancy between the two the test with 

 the coloured lights should be accepted; for it is usually the normal 

 perception and discrimination of coloured lights which has practical 

 importance. 



20. Talbot's Law. Rotate a disc one sector of which is black and the 

 rest white, or a disc like that in Fig. 446 (p. 1050). A uniform shade is 

 produced as soon as a speed of about 25 revolutions a second has been 

 attained, and this is not altered by further increase in the speed. 



21. Purkinje's Figures. (a) Concentrate a beam of sunlight by a 

 lens on the sclerotic at a point as far as possible from the corneal margin, 

 passing the beam through a parallel-sided glass trough filled with a 

 solution of alum to sift out the long heat-rays. The eye is turned 

 towards a dark ground. The field of vision takes on a bronzed appear- 

 ance, and the retinal bloodvessels stand out on it as a dark network, 

 which appears to move in the same direction as the spot of light on the 

 sclerotic. A portion of the field corresponding to the yellow spot is 

 devoid of shadows (p. 1043). 



(6) Direct the eyes to a dark ground while a flame held at the side of 

 the eye, and at a distance from the visual line, is moved slightly to and 

 fro. A picture of branching bloodvessels^appears. This experiment 

 is performed in a dark room. 



(c) Immediately on awaking look at a white ceiling for an instant; 

 a pattern of branched bloodvessels is seen. If the eye be at once closed, 

 and then opened with a blinking movement, this may be observed again 

 and again. Ultimately the appearance fades away. 



HEARING, TASTE, SMELL, TOUCH, ETC. 



22. Monochord. Study by means of the monochord, a stretched 

 string with a movable stop, the relation between the pitch of the nqte 

 given out by a vibrating string, and its length and tension. 



23. Beats. Cause two tuning-forks of nearly equal pitch to vibrate at 

 the same time. Make out the beats, and count their number per second. 



24. Sympathetic Vibration. Take three tuning-forks mounted on 

 resonators. Let two of them be of the same pitch. Strike one of 

 these; the other is thrown into sympathetic vibration, and continues 

 to give out a note after the first is quickly stopped by touching it. 

 The third fork is unaffected. 



25. Determine by means of Galton's whistle the pitch of the highest 

 audible tone. 



26. Cranial Conduction of Sound. When a tuning-fork is held 

 between the teeth, a part of the sound passes out of the ear from the 

 vibrating membrana tympani; if one ear is closed, the sound is heard 

 better in this than in the open ear. If the tuning-fork is held between 

 the teeth, till, with both ears open, it becomes inaudible, it will be 

 heard for a short time if one or both ears be stopped ; and when in this 

 position the sound again becomes inappreciable.it can still be caught 

 if the handle be introduced into the auditory meatus. 



27. Taste. (i) Apply to the tongue by means of a camel's-hair 

 brush a solution of quinine (i to T.OOO), sodium chloride (i to 200), 

 cane-sugar (i to 50), and sulphuric acid (i to 1,000). Determine at 

 what part of the tongue the strongest sensations are elicited by each. 



