326 ON LIGHT. 



pleasing appearances will be witnessed. To see them to 

 the greatest advantage, a magnifying glass should be 

 used, placing the eye in the place of the screen, and looking 

 through the glass at the fringes and images of the holes as if 

 they were real objects in its focus. 



(109.) When the system of apertures examined con- 

 sists of a great multitude of exceedingly narrow parallel 

 slits, precisely equal and equidistant, they constitute 

 what is called a " diffractive grating," and present very 

 curious, and in some cases brilliant, phenomena, which 

 are best viewed by placing the eye close behind the grat- 

 ing. The luminous point (which appears colourless) is 

 then seen accompanied laterally and on either side by a 

 succession of highly coloured spectra, arranged in a line 

 passing through it, and with their lengths directed along 

 that line ; their colours, unlike those of the fringes 

 (which are composite) are the pure unmingled hues of 

 the prismatic spectrum : even more vivid (if the grating 

 be delicately executed) than the best spectrum which 

 can be formed by refracting a sunbeam through a prism; 

 and exceedingly remarkable in another respect, viz., that 

 the proportional lengths of the coloured spaces in each, 

 instead of depending, as in the case of the spectrum 

 formed by a prism, on the nature of the particular 

 medium of which the prism consists, is independent of 

 any such consideration, and determined solely by the 

 proportion between the wave-lengths corresponding to 

 the colours of the rays. They are, therefore, what may 

 be called normal spectra. So pure and undiluted indeed 

 are their tints, that by the aid of a magnifying telescope 



