36 



LECTURES TO SCIENCE TEACHERS. 



If a screen were placed exactly in the focus, and white 

 light containing light of all shades of refrangibility were 

 allowed to fall on the prisrn, each point of the screen whicli 

 was illuminated at all would be illuminated by one kind of 

 light only ; the different kinds would be separated from one 

 another on the screen, and consequently you would get a pure 

 spectrum, and that without the tremendous loss of light 

 encountered if you employ two holes placed at a distance 



FIG. C. 



from each other. The spectrum thus formed, though pure, 

 would be infinitely narrow ; but in order to give it breadth 

 you have only to substitute a line of light (in a direction 

 parallel to the edge of the prism) from the point of light ; in 

 other words, to transmit the light in the first instance through 

 a narrow slit instead of a small hole. This is the way in 

 which a pure spectrum is generally formed as a matter of 

 principle, though sometimes a mirror is used instead of a lens ; 



