410 CHROMATICS SPECTRA. 



dust. If a prism be placed in the path of the beam, as 

 shown in Fig. 314, the sides of the slit and edges of the 

 prism being horizontal, the beam will be refracted upward. 

 If the refracted beam be caught upon a screen, it will 

 appear as a band of differently colored light, passing by 

 imperceptible gradations from red at the bottom, through 

 orange, yellow, green, blue and indigo to violet at the 

 upper end of the beautifully colored band. This colored 

 band is called the solar spectrum. 



(a.) The different colors do not occupy equal space in the spectrum, 

 orange having the least and violet the most. The initials of these 

 colors form the meaningless word VIBGYOR, which may aid the 

 memory in remembering these prismatic colors in their proper 

 order. By placing the slit in a vertical position, and standing the 

 prism on its end so that its edges will be parallel with the sides of 

 the slit, the spectrum will be projected as a horizontal band. 



636. Dispersion. By looking at Fig. 314, it will 

 be seen that the red rays have been refracted the least and 

 the violet the most of all the luminous rays. This sepa- 

 ration of the differently colored rays by the prism 

 is called the dispersion of light ; it depends upon the 

 fact that rays of different colors are refracted in different 

 degrees. 



637. Pure Spectrum. The spectrum above de- 

 scribed is composed of overlapping and differently-colored 

 images of the slit. In a pure spectrum these images must 

 not overlap. The first requisite in preventing this over- 

 lapping is that the slit be very narrow. 



(a.) The most simple way of producing a pure spectrum is to look 

 through a prism at a very narrow slit in the shutter of a darkened 

 room. The edges of the prism should be parallel to the slit ; the 

 prison should be at least five feet (1 m.) from the slit ; the prism 

 should be turned until the colored image of the slit is et the least 



