138 THE ART OF PROJECTING. 



lens that the beam is again crossed in front of it, and 

 the disk upon the screen is of the desired size, six or 

 eight feet in diameter. Now introduce the perforated 

 paper or the eidotrope at the place marked e. The 



Fig. lOr,. 



screen will appear covered with minute spectra, as 

 each hole will form one or more spectra ; but if the 

 paper be held at e, between the lens and the screen, the 

 projection becomes very gaudy and symmetrical. If it 

 be the eidotrope, turn it while held in that place, and 

 the colors will change and will rival the colors pro- 

 duced by polarized light. Try the effect of a comb, 

 of wire gauze, of the fingers, or other objects. Very 

 curious and interesting appearances will appear upon 

 the screen. 



If one has a piece of glass finely ruled with a dia- 

 mond, it may be projected as is any object with the 

 porte lu7niere^ and the diffraction spectra will appear 

 upon the screen. With plenty of light for the projec- 

 tion, and with the room otherwise well darkened, a 

 number of the Fraunhofer lines may be seen in these 

 spectra. 



Again, let a concentrated solution of alum or cam- 

 phor be poured upon a glass plate, and allowed to dry 

 rapidly, so as to cover it with a crust. Put it in the 

 focus of a lens with a short focus, and a series of 



