340 



SCIENTIFIC AMUSEMENTS. 



Fig. 187. Real Image formed 

 by Convex Lens. 



magnified upwards of 100 times in linear dimensions, or upwards 

 of 10,000 times in area. 



Expt. 368. The Magic Lantern. The simplest form of magic 

 lantern consists of an illuminated object, AB (fig. 187), placed at a 

 distance from a convex lens only a little greater than its focal 

 length, in which case a magnified image, ab, will be formed on the 

 other side of the lens, further differing from that produced by the 

 ordinary magnifying glass in being inverted and also real, so that 

 it can be received on a screen, such as a strained wet sheet, &c. If 



A the distance of the object from the 

 lens be somewhat increased, that of 

 the image when clearly defined will 

 be proportionately decreased, as also 

 will the degree of magnification; if 

 the distance of the object be in- 

 creased to twice the focal length, 

 that of the image will diminish to 

 the same value, and the object and 

 image will be of the same size; 

 whilst if the object be still further 

 removed, the image will be formed 

 at a distance from the lens greater 

 than the focal length, but less than twice that amount, and will 

 now be smaller than the object (vide Expt. 370). 



The better class of magic lanterns are illuminated by means of 

 lime or electric lights, so that a transparent slide (e.g., a photograph 

 on a glass plate) can be very largely magnified without being 

 rendered too faint owing to insufficient light. Specially arranged 

 combinations of lenses are also employed to get rid of spherical and 

 chromatic aberration (Expts. 361, 365), and to obtain a " flat field," 

 i.e., to ensure, as far as possible, that the outer portions of the 

 image on the flat screen may be " focussed " and sharply defined 

 equally with the central part. Cheaper lanterns with ordinary 

 uncompensated lenses usually show defects of this kind to a greater 

 or lesser extent; oil lamps are generally used as the source of 

 light in such instruments, whence very highly enlarged pictures 

 cannot be obtained, owing to lack of illuminating power. 



Dissolving views are produced by having two lanterns side by 

 side so arranged that one projects an image on the screen, whilst 

 an opaque cap or shade cuts off the light of the second. This shade 

 is so arranged that by slowly moving it the light from the first 

 lantern is gradually cut off, and that from the second allowed to 

 reach the screen, so that the one picture gradually fades as the 

 light producing it is cut off, whilst the other appears at first 



