120 THE ART OF PROJECTING. 



focus in front of the lantern the slit d. The objective 

 o may then be so placed as to project a sharp image of 

 the slit upon a screen in front of it at a distance of 

 fifteen or twenty feet ; then place the triangular prism 



Fig. 92. 



close to the objective. The screen will now need to 

 be moved, that the refracted rays may fall upon it, and 

 at the same distance from the objective that it stood 

 in front, otherwise the edges of the spectrum will 

 appear blurred. This should give a spectrum about 

 five feet long at the distance of twenty feet, but the 

 length will depend upon the dispersive power of the 

 prism. It will be longer with a bisulphide of carbon 

 prism than with one made of glass. If a still longer 

 one is needed use two similar prisms close together 

 and each one turned to the point of minimum devia- 

 tion. 



If a very pure spectrum is needed, all of the con- 

 densers may be removed and the slit put in their place. 

 A parallel beam will then fall upon it, and the projec- 

 tion may then be made in precisely the same way as 

 for the solar spectrum. In this case the light will be 

 much less intense. 



2d. To project the spectrum of the elements : 

 Remove the lime cylinder and its holder, and light 

 the gases : the tongue of flame will be six or eight 



