CUR OMA TICS SPECTRA . 417 



neutralizing its refraction. As the converging effect of the 

 compound lens is not destroyed, images may be formed ; 

 as the dispersive effect is destroyed, the colored fringe is 

 avoided. A co^nbinatio1^ of lenses by ivhich disper- 

 sion is avoided and refraction secured is called an 

 achromatic lens. 



648. Properties of the Spectrum. We have seen that 

 we may decompose a sunbeam by availing ourselves of the varymg 

 refrangibility of the different kinds of rays of which it is composed. 

 We have been able in this manner to produce the seven primary 

 colors from white light. But our analytic investigations must go 

 still further. Beyond the limits of the visible spectrum, in both 

 directions, there are rays that do not excite the optic nerve, the ex- 

 istence of which, however, may be easily proved. The spectrum 

 has three properties which we must consider in detail : luminous, 

 thermal and actinic. 



649. Luminous Spectrum. We have seen the 

 difference of rays in different parts of the spectrum in re- 

 gard to color, but they also differ in respect to intensity or 

 illuminating power. An object, as a printed page, will be 

 illuminated more strongly, and therefore seen more dis- 

 tinctly, when placed in the yellow than when placed in the 

 red or violet part of the spectrum. 



(a ) The difference in color between the rays found in different 

 parts of the spectrum is merely one of rate of vibration or wave- 

 length. The intensity of the light found at any particular part of 

 the spectrum depends upon the amplitude of vibration. In respect 

 to the visible spectrum, it may be said that brightness is to light 

 what loudness is to sound ( 431), that color is to light what pitch 

 is to sound ( 434). 



(&.) The length of an ether- wave that can awaken the sensation of 

 redness is about OTTO* f an * nc h > ^ t l iat which can awaken th 

 sensation of violet, about ^^in;* The waves corresponding to the 

 intermediate colors have intermediate lengths. 



650. Thermal Spectrum. If a very delicate 

 thermometer or thermopile be successively placed in vari- 



