

THE SPECTRUM OF WHITE LIGHT. 487 



fracting edge is directed upwards, and therefore the 

 deviation takes place downwards. 



Strictly speaking, the spectrum of white light is com- 

 posed of an infinitely great variety of colours, each one 

 being very little different from the next in order of 

 succession ; although a sharp separation is thus impos- 

 sible, on account of the gradual transition of one colour 

 into the next, certain principal groups have been dis- 

 tinguished by the following names, beginning with the 

 colour of the least refrangible rays : 



Red 



Orange 



Yellow 



Green 



Blue 



Indigo 



Violet 



If a spectrum produced by the carbon disulphide 

 prism above described, be received on a wall 2 m distant, 

 the dispersion would lengthen a spot 5 cm in diameter 

 to a band 20 cm long and 5 cm wide, in which the single 

 colours still overlap for a space of nearly 5 cm , so as to 

 produce a mixture in the middle. 



A purer spectrum is obtained if a narrow slit instead 

 of a circular aperture is interposed in the path of the 

 rays before they reach the prism. This may be accom- 

 plished in various ways, and it is then best to view the 

 spectrum direct that is, to look through the prism at 

 the slit. 



A slit, 5 cm long and 5 or 6 mra wide, may be cut in the 

 middle of a large sheet of pasteboard not less than 



