VISION 183 



For water, 



sineJ4 



sine r 3 



(4) To determine the index of refraction of glass proceed as in 

 the case of water. Set the instrument upon the table; the block of 

 glass may be placed upon the body of the instrument, the polished 

 surfaces being placed above and below. If the distance between 

 the polished surfaces is not equal to x y, a point (?/') may be located 

 on the upper surface near the edge of the glass block by making 

 a dot with ink where the line x y cuts the upper surface of the block. 

 This line is the normal. 



What is the index of refraction of the glass block furnished by 

 the demonstrator? 



IV. TO DETERMINE THE FOCAL DISTANCE OF A LENS; THEN 

 THE USE OF THE FORMULA 



1 L-1--1 



o "*" i ~~ F' 



An easy method of determining the focal distance of a lens 

 depends upon the relation of the distance of the conjugate foci to 

 the general focal distance. This relation may be expressed thus: 

 The sum of the reciprocals of the conjugate foci is equal to the 

 reciprocal of the focal distance. 



Now, when a lens throws upon a screen the image of an object 

 it is evident that the distance of the object (o) represents one and 

 the distance of the image (i) represents the other of these conjugate 

 focal distances; so one may say: The reciprocal of the distance of 



the object from the lens (-) plus the reciprocal of the distance of 



the image (-) equals the reciprocal of the general focal distance ( ) ; 

 i F 



thus ( -f-_= ).. This formula enables one to compute the focal dis- 

 tance after first determining by experiment the values o and i. 



1. Apparatus. To that end one may construct a simple apparatus 

 (Fig. 77). For the determination of the focal distance it is usual 

 to have both object and lens movable. For our purpose this may 

 be dispensed with, as it lends little to the reliability of the result 

 and detracts much from the simplicity of the apparatus. Construct 

 from half-inch pine boards a box 100 cm. or 50 cm. long and about 

 8 cm. high and wide (inside measurements). The box should be 

 open at one side. The inner surface of one end may be painted 

 white and serve as a screen ; the other end should have in its center 

 a large hole. Over this hole, on the inner surface of the upright, 



