35 



into it.. Gradually the mark on the paper becomes distinctly 

 visible. Is it erect or inverted ? (The observer's eye must be 

 kept accommodated for distant vision.} 



Indirect Method. With the mirror about three feet from the 

 model eye reflect the light into it. Now insert a biconvex lens 

 at about 4 or 5 inches in front of the model, and try to see 

 the image of the mark on the paper. Is it erect or inverted ? 



The human eye may be examined in the same way. 



B. Vision with One Eye. 



I. The Focussing Mechanism. 



I. The Formation of Images in the Retina. 



METHODS. A. Study the formation of images on an 

 obscure glass screen behind a lens as to 



1. Where an object held above is focussed on the screen 

 and where an object on the right is focussed. Is the image 

 erect or inverted ? 



2. Can a near object and a far object be focussed at the 

 same time ? 



3. What is the relationship of the size of the image to the 

 distance of the object ? 



Revise your knowledge of the optical properties of a convex 

 lens. 



B. Examine the image of a candle formed on an obscure 

 glass plate placed over a hole cut in the back of a fresh eye 

 of an ox. 



II. At what Surfaces of the Eye are the Rays of Light 

 refracted ? 



Note the formation of a reflected image from each of 

 the two refracting surfaces of a biconvex lens. Study, (1) how 

 the size of the image varies with the curvature of the lens, 

 (2) the position, erect or inverted, of the image from each 

 surface of the lens, and (3) the direction in which it moves 

 when the object is moved. 



METHOD. Sanson's Images. In a dark room hold a 

 candle to the outer side of the eye of a fellow student, and 



