166 



LIGHT AND SOUND 



Eye ftece ' 



then pass through the object lens, and produce a reversed and inverted 

 image near the eye-piece. Just as the simple magnifying lens enlarges 

 an object placed near it, so the eye-piece enlarges the 

 image a b f , and it appears to be further magnified to 

 the size a" b" (read this: a second, b second). 



188. The Camera. The camera (Fig. 

 160) consists of a dark box having a movable 

 screen of ground glass (). The tube con- 

 tains a convex lens. The image of the object 

 appears upon the screen, and may be looked 

 at through 0. When the image has been 

 " focused " on the screen, a " sensitized " 

 film or glass plate is put in place of the 

 screen. The chemical action of light 

 " prints " a picture of the object upon the 



film Or plate. 



. , , 



The sensitive film or plate is made by 



. 



covering a sheet of gelatine or of glass with 

 some compounds that are easily decom- 

 posed by light. Usually these are silver chloride or silver 

 bromide. After the film or plate has been exposed to light 

 from the object, the image is still invisible. It must be 

 " developed" by means of a solution of pyrogallic acid, 

 or some other " developer." Finally the image is " fixed." 

 Fixing consists in removing 



the unchanged silver salt, j S |J ^ _ ^^^ 

 The material used is generally o ^^^^3^E: 

 sodium thiosulphate, known J f| |p^ be 

 popularly as " hypo." FIG. ieo. 



The camera is a device for focusing the 

 , , , , . . clear image of an object upon a sensi- 



The result of these operations is t ive plate or film. 



Objective 



The compound 

 microscope makes 



an object appear 



much larger, but 



