VISION. 203 



axis of the lens carrier. The screws S and S' serve the 

 double purpose of protecting the projection (p) from 

 splitting off and of affording handles by which the car- 

 rier may be slipped along the groove. Along one edge 

 of the groove on the outer surface of the bottom make a 

 centimeter scale carefully with a sharp hard lead pencil. 

 The scale should have its zero point in the plane of the 

 screen. At the point D fix a shaft (such a one as shown in 

 Fig. 27, D'), which shall extend several centimeters below 

 the bottom and set perpendicular to it. The shaft may 

 be fixed in a universal clamp-holder and the whole sup- 

 ported upon a heavy support. By adjusting the clamp- 

 holder the apparatus may be directed toward any desired 

 object. Make a cover to the box, and blacken the whole 

 inside. 



3. Observations. Fix a lens in place; close the box; direct 

 its axis toward some well illuminated distant object; 

 grasp the handles of the lens carrier and move it to a 

 position which gives upon the screen a sharply defined 

 image of the object in the field. One has only to read 

 the position of the transverse line of the carrier on the 

 centimeter scale to have the focal distance of the lens; 

 i. e., the distance at which parallel rays are focused, 

 c. Verification of the formula 1 -f 1 = i. 



A second method of determining the focal distance of a 

 lens depends upon the relation of the distances of the conju- 

 gate 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 



