LXXl.] FORMATION OF IMAGE. 329 



PHYSIOLOGY OF THE SENSE ORGANS. 



LESSON LXXI. 



FORMATION OF IMAGE — DIFFUSION — ABER- 

 RATION — ACCOMMODATION — SCHEINER'S 

 EXPERIMENT — NEAR AND FAR POINTS— 

 PURKINJE'S IMAGES— PHAKOSCOPE— ASTIG- 

 MATISM— PUPIL. 



1. Formation of an Inverted Image on the Retina. 



(a.) From the fresh excised ox-eye remove the sclerotic from 

 that part of its posterior segment near the optic nerve. Roll up a 

 piece of blackened paper in the form of a tube, black surface inner- 

 most, and place the eye in it with the cornea directed forwards. 

 Look at an object — e.n.^ a candle-flame — and observe the inverted 

 image of the flame shining through the retina and choroid, and 

 notice how the image moves when the candle is moved. 



[h.) Focus a candle-flame or other object on the ground-glass plate of an 

 ordinary camera for photographic purposes, and observe the small inverted 

 image. 



{c.) Fix the fresh excised eye of an albino rabbit in Du Bois-Reymond's 

 a])paratus provided for you, and observe the same phenomenon. The eye is 

 fixed with moist modeller's clay. Observe the effect on the retinal image 

 when a convex or concave lens is placed in front of the cornea. These lenses 

 rotate in front of the cornea, and are attached to the instrument. 



2. Diffusion. 



{a.) 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. 



{h.) Prick a smooth hole in a card with a needle, arrange the 

 needle at the proper distance to obtain the previous diffusion eff'ect, 

 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 diffusion circles are cut off, and larger because the 

 object is nearer the eye. 



