154 ELEMENTS OF LABORATORY WORK 



We may measure the distance from the lens of the image, 

 which is produced by the light from an object passing through 

 the lens, by means of an optical bench (fig. 52). This consists 

 of a long wooden horizontal stand, along which may be moved 

 vertical stands holding the luminous object, the lens, and a 

 screen of white unglazed paper to receive the image. Each of 

 these stands is made so that the uprights may move perpen- 

 dicularly across the long scale A of the bench, which is shown 

 in section at x. The lateral movement is obtained by a block 

 sliding in a groove, as shown at c, in Y and z. The longi- 

 tudinal movements of the stands are measured by a stand 

 carrying a horizontal wire, the length of this wire being known 

 and added to the distance through which its stand is moved in 



taking a measurement. Screws at D and E enable the screens, 

 lenses, or mirrors to be adjusted in height. Good observations 

 with lenses and mirrors can be made on such a bench. 



In the stand for the luminous object place a screen with a 

 central aperture and a pin vertically fixed with its point at 

 about the centre of the hole. Close behind this place a strong 

 light. The lens and screen are now adjusted until a clear 

 image of the pin-point is obtained. It will be found that for 

 every change in the distance of the luminous object from the 

 lens there is a change in the distance of the image from the 

 lens. The points thus found are the conjugate foci. 



If the light from a distant object be made to pass through 

 the lens, it will be found that the image formed has an invari- 



