PRACTICAL EXERCISES 863 



Fig. 334, so that a part of each is seen. On another arrange a violet 

 and a yellow disc, and on the third an orange and a blue disc. By 

 adjustment of the proportions of the two colours a uniform grey can 

 be obtained from each of these combinations (complementary 

 colours) when the discs are rapidly rotated. 



(b) Mix two colours that are not complementary, e.g., blue and 

 red ; grey or white cannot be obtained by any adjustment of pro- 

 portions ; the result is always a mixed colour, the precise hue 

 depending on the amount of each ingredient. 



(c) Take papers of any three colours from widely-separated parts of 

 the spectrum, e.g. t blue, green, and red, and arrange them on one of 

 the rotating discs. By varying the proportions white can be produced, 

 and any other coloured paper fastened on another of the rotating 

 discs can be matched by adding white to the three colours. 



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

 the rest white, or a disc like that in Fig. 315. 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. 



10. 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 ray 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. 810). 



(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 (p. 811). 



(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. 



11. 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. 



12. 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. 



13. Aesthesiometry. Measure on different parts of the skin and 

 accessible mucous membranes the distances at which the points of 

 a pair of compasses must be held apart in order that two distinct 

 sensations may be experienced (p. 851) (aesthesiometer). 



