optics. 317 



nifying paintings on glass, and throwing their 

 images upon a white screen in a darkened chamber. 

 But it is capable of being employed for more im- 

 portant purposes, by using such figures as will ex- 

 plain the principles of astronomy, botany, &c. Its 

 construction is very simple ; it consists of a tin 

 lanthorn, within which is a lamp, whose light passes 

 through a great plano-convex lens placed in a tube 

 fixed in the front. This strongly illuminates the 

 small transparent painting on glass placed before 

 the lens in an inverted position ; another tube, 

 containing a convex lens, slides within the other, 

 so as to adjust the focal distances of the glasses. 

 The illumination is often increased by means of a 

 convex mirror placed at the back of the lamp. To 

 render the picture distinct, no light should fall 

 upon it but what passes through the lens. 



The exhibition called the Phantasmagoria, which 

 lately excited so much admiration, is nothing more 

 than a magic lanthorn constructed in a peculiar 

 manner. In the common lanthorns, the figures are 

 painted on the glass, but all the rest of the glass is 

 left transparent ; consequently, the image on the 

 screen is a circle of light having a figure on it. 

 But in the phantasmagoria, all the glass is made 

 opaque, except the figure only, which being painted 

 in transparent colours, the light shines through it : 

 for this reason, therefore, no light can come upon 

 the screen l>ut what passes through the figure 

 itself; consequently, you have upon the screen a 

 figure only, without any circle of light as in the 

 common magic-lanthorn. 



Instead, also, of the representation being made 

 upon a wall or a sheet, as is usually the case, it is 

 thrown upon a thin screen of silk placed between 



