CAMERA OBSCURA. 341 



faint and subsequently stronger, till at length the first picture is 

 entirely obliterated, all light from the first lantern being cut off ; 

 whilst the second picture is visible to the fullest extent, the shade 

 being now wholly removed from the front of the second lantern. 

 Instead of using a shade, the same result can be obtained by turn- 

 ing the lights in the two lanterns up or down as required. For 

 certain kinds of effects both lanterns may be used simultaneously, 

 or even more than two lanterns, one being used to produce part of 

 the effect, the second another part, and so on; e.g., a phantom 

 figure may be projected on the screen by the aid of one lantern, 

 whilst the rest of the scene is shown by the second. By suddenly 

 putting a shade in front of the first lantern, or by turning down 

 the light therein, the phantom disappears, and so on. 



In the solar microscope the source of light is the sun's rays 

 reflected from a mirror, the general construction of the rest of the 

 instrument being like that of the ordinary magic lantern. 



Various instruments are also sold under different names, whereby 

 the image of a highly illuminated solid (not transparent) object is 

 projected on the screen, the formation of the image by lenses being 

 formed in just the same way as with the ordinary lantern, but the 

 lighting arrangements and other details being different, according 

 to the circumstances of the case. 



Expt. 369. To determine the Focal Length of a Convex Lens. 

 Arrange a lighted candle directly in front of the lens, so that 

 the centres of the lens and flame may be in line, the light falling 

 full upon one face of the lens, which should be held in position 

 by a convenient clamp or other stand. Slightly grease a piece of 

 note paper, and hold this behind the lens so as to receive the 

 image of the flame, which will appear inverted and will be magnified 

 or diminished according as the paper screen is further from or 

 nearer to the lens than the candle. Now move the candle nearer 

 to or further from the lens, along the direct line joining them, also 

 moving the screen, until finally such positions are arrived at that 

 whilst the inverted candle flame image is clear and distinct, the 

 screen and candle are at equal distances from the lens. This 

 happens when each is at a distance from the lens equal to twice 

 the focal length, whence it follows that the focal length is one- 

 fourth of the distance between the candle and screen. 



Expt. 370. The Camera Obscura. The camera obscura is opti- 

 cally the converse of the magic lantern, consisting in its simplest 

 form of a lens so placed with reference to a receiving screen and 

 surrounding objects that the distance of the latter from the lens is 

 great as compared with that of the former, so that a diminished 

 and inverted real image is formed. With the ordinary 



