998 A MANUAL OF PHYSIOLOGY 



Measure the distance from the card at which this occurs, and calcu- 

 late the size of the retinal image (p. 904) . 



19. Colour-blindness. Spread out Holmgren's coloured wools on 

 a sheet of white filter-paper in a good light. Do not mention the 

 colours of any of the wools, but (i) ask the person who is being 

 tested to pick out all the wools which seem to him to match a pale pure 

 green wool (neither yellow green nor blue green), which is handed to 

 him. He is not to make an exact match, but to pick out the skeins 

 which seem to have the same colour. If he makes any mistakes, 

 by selecting, e.g., in addition to the green skeins any of the ' confusion 

 colours,' such as grey, greyish yellow, or blue wools, there is some 

 defect of colour discrimination. To determine whether the person 

 is red or green blind, tests (2) and (3) are then made. (2) Give him 

 a medium purple (magenta) wool, and ask him to pick out matches 

 for it. If he is red-blind, he will select as matches to it only blues 

 and violets, as well as other purples. If he is green-blind, he will 

 select only greens and greys. (3) The third test is a red wool. In 

 selecting matches for this, the red-blind will choose (with reds) greens, 

 greys, or browns less bright 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. 412 (p. 938) . 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 

 appearance, 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. 930). 



(b) 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 

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



