122 THE ART OF PROJECTING. 



Lithium, and Copper are especially good for this work 

 and give satisfactory spectra. 



When this monochromatic light from the stick of 

 glass or the saturated solution of sodium chloride is 

 made to appear, it will be a good time to give atten- 

 tion to its effects upon other colors. Observe the 

 faces of individuals, the colors of flowers, of ribbons, 

 of pictures. It is a good plan to have prepared a set 

 of strips of bright-colored papers, or ribbons, or the 

 Newton's disk, for exhibition in monochromatic light. 



REVERSED LINE. 



The dark sodium line is the only one that is ever 

 projected, owing to the great difficulty there is in 

 making the vapors of other substances sufficiently 

 dense to absorb the powerful rays from the electric arc 

 or of the lime light. With either, a pure spectrum 

 must first be projected, and the slit should be nicely 

 focussed, as described. Then having provided a 

 gas jet with Bunsen burner, or an alcohol lamp in 

 front of the slit, hold in it a small iron spoon con- 

 taining a lump of metallic sodium as large as a pea. 

 It will take fire and burn with a yellow blaze and a 

 white vapor, through which the light from the lantern 

 must pass* If this vapor is dense enough it will stop 

 rays from the other light that have the same refrangi- 

 bility ; and as its own luminousness is not very great, 

 it will leave a black line upon the screen in the place 

 where the sodium line would appear if the light came 

 from it. 



It will be best to have a screen a foot square with a 

 hole through it, to set in front of the sodium flame to 

 prevent its light from falling upon the large screen and 

 injuring the effect. 



