170 THE ART OF PROJECTING. 



kind of sound. The changes in the patterns, combined 

 with the many colors of the film, make interesting 

 studies in acoustics. It is important that the tube upon 

 which the films are made should be fixed while the 

 projections are looked at, for otherwise the coruscations 

 cannot be seen. It will be sufficient, however, to 

 fasten it in a retort holder. As the light for the pro- 

 jection is reflected from the surface, it will be best not 

 to have the reflected beam more than about ninety 

 degrees from the incident beam, otherwise the pro- 

 jection will appear too oval, it will also be less distinct. 



LANDSCAPE PROJECTION. 



In the experiments to illustrate rectilinear move- 

 ment of light, on page 81, it is remarked that the 

 appearance of the landscape as shown on the walls of 

 the room is much brighter when snow is upon the 

 ground. The definition is made much better by making 

 the orifice small, but then the light is so much scattered 

 that the images are not very distinct. By employing a 

 lens with as long a focus as possible, and placed at the 

 orifice, a beautiful image of the external landscape will 

 appear upon a screen at a proper distance. A lens 

 with almost any focal length will show good definition, 

 but the projection will be too small for any considerable 

 number to see at once. A lens with six or eight feet 

 length of focus will show a picture five or six feet 

 square with all the details of the landscape easily 

 discernible in a darkened room. 



SODIUM LINE IN SOLAR SPECTRUM. 



Having arranged the apparatus as shown in Fig. 

 89, for showing the more prominent Fraunhofer lines, 



