330 PRACTICAL PHYSIOLOGY. [LXXI. 



(''•.) In a dark room place a lighted candle or gas-bnrHer con- 

 veniently, and by means of a convex lens focus the image of the 

 flame on a sheet of white paper. It is better to introduce a 

 blackened cardboard screen with a narrow hole in it between the 

 light and the lens. Observe that a sharp image is obtained only 

 at a certain distance from the lens. If the white screen be nearer 

 or farther away, the image is blurred. 



3. Spherical Aberration. 



jNIake a hole in a blackened piece of cardboard with a needle, 

 look at a light placed at a greater distance than the normal distance 

 of accommodation. One will see a radiate figure, with four to 

 eight radii. The figures obtained from opposite eyes will probably 

 differ in shape. 



4. Chromatic Aberration. — Coloured Fringes. 



(a.) With one eye fix steadily the limit between a white and 

 black surface (e.g., fig. 265), and while doing so bring an opaque card 

 between this eye and the object (the other eye being closed), I^t 

 the edge of the card be parallel to the limit between the white and 

 black surfaces, so as to cover the larger part of the pupil. The 

 margin next the black appears with a yellowish-red fringe when 

 the part of the pupil which lies next the black surface is covered, 

 while there is a bluish-violet fringe in the opposite condition. 



(b.) Make a pin-hole in a blackened card, and behind the hole 

 place a cobalt glass. Look at a gas-flame through this arrangement. 

 The cobalt glass allows only the red and violet rays to pass through 

 it. Accommodate for the violet rays or approach the light, the 

 flame appears violet, surrounded with a reddish halo ; on accommo- 

 dating for the red, or on receding, the centre is reddish with a 

 violet halo. 



(c.) Place a strip of red paper and one of blue on a black surface. 

 The red appears nearer than the blue, because one makes a greater 

 effort to accommodate for the less refrangible red rays than for the 

 more refrangible blue or violet, and hence the red is judged to be 



{d.) V. Bezold's Experiment. — Make a series (10-12) ot concentric circles, 

 black and white alternately, each i mm. thick, the diameter of the whole 

 being about 1 5 mm. On looking at these circles when they are placed within 

 the focal distance, one sees the white become pink ; to some eyes it a])pears 

 yellow or greenish. The same is seen on looking at concentric black and 

 white circles, or parallel black and white lines from a distance outside the far 

 point of vision ; the white appears red and the black bluish. 



{e. ) Wheatstone's Fluttering Hearts. — (i. ) Make a drawing of a red-coloured 

 heart on a bright blue ground. In a dark room lighted by a candle hold the 

 picture below the level of the eyes, and give it a gentle to and fro motion. 



