THE ART OF PROJECTING. 



A MAGNIFIED image of a picture, or of any phenom- 

 enon, when thrown upon a screen by means of sunlight, 

 and lenses, or with a magic lantern, is called a projec- 

 tion. 



When sunlight is to be used for this purpose, it is 

 necessary to have some fixture to give the proper direc- 

 tion to the beam. The heliostat and the porte lumiere 

 are the devices in common use. The latter was the 

 earliest form, and was invented by Gravesand, a Dutch 

 professor of natural philosophy, in the early part of the 

 last century. It was afterwards reinvented by Captain 

 Drummond, an Englishman, who called it the heliostat. 

 The latter term is now only applied to an automatic 

 arrangement, by which a mirror is moved by clock- 

 work in such a way that a beam of sunlight reflected 

 from it may be kept in one direction all day, if it be 

 needed so long. Silberman and Foucault have each 

 devised very satisfactory instruments, but they are too 

 costly to be owned by any but the wealthy ; the catalogue 

 price of the "cheapest of these being five hundred 

 francs. C. Gerhardt, of Bonn, however, makes a small 

 one, carrying a good mirror three inches in diameter, 

 for twenty dollars. 



