264 EXPERIMENTAL PHYSIOLOGY. 



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 further away, the 

 image is blurred. 



(&.) Fix a long needle in a piece of wood, or use a pencil 

 or penholder, close one eye, and bring the needle or pencil 

 gradually nearer to the other eye. After a time, when the 

 needle is five to six inches distant, it will no longer be 

 distinct, but blurred, dim, and larger. 



(c.) Prick a smooth hole in a card with a needle, arrange 

 the needle at the proper distance to obtain the previous 

 diffusion effect, and now introduce the card between the 

 needle and the eye, bringing the card near the eye, and 

 looking through the hole in the card. The needle will 

 appear distinct and larger it is distinct because the diffu- 

 sion circles are cut off, and larger because the object is 

 nearer the eye. 



3. Spherical Aberration. 



(a.) Make with a needle a hole in a blackened piece of 

 cardboard, 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 ob- 

 tained from opposite eyes will probably differ in shape. 



4. Chromatic Aberration. Coloured Fringes. 



(a.) Fix steadily the limit between a white and black 

 surface, and while doing so bring an opaque card between 

 one eye and the object (the other eye being closed), with its 

 edge 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 yellow fringe when 

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

 covered, while there is a blue fringe in the opposite condition. 



(6.) Look at a candle or gas-flame through a small hole 

 in a black card with cobalt glass placed behind the hole, 



