CHR OMA TICS SPECTRA. 



419 



652. Curves of Intensity. Fig. 323 represents, by means 

 of curves, the relative intensities of these three properties of the 

 solar spectrum produced by a flint-glass prism. The wave-length 

 will determine the position of the ray in the horizontal band. The 

 greatest heating effect will be found just beyond the red rays ; the 

 highest point of the thermal curve is over this part of the spectrum. 

 The greatest illuminating effect will be found a little at the right of 

 D ( 638) ; the highest point of the luminous curve is over this point. 

 The position of greatest actinic effect is similarly indicated. If a 

 rock-salt or a quartz prism be used, the curves will be somewhat 



DARK HEAT RAYS 



CHEMICAL RAYS, 



FIG. 323. 



different from those here shown. But it has been found that when 

 a spectrum is produced, the dispersed rays are distributed very 

 unevenly. Many more rays are crowded into the red end than into 

 the violet end. While, therefore, one part of the spectrum may 

 show a greater heating effect than another part, it does not follow 

 that a single ray of low refrangibility has greater heating power 

 than a single ray of high ' refrangibility. The researches of Dr. 

 Draper, of New York, go to show that one ray has a heating effect 

 equal to that of any other ray in the spectrum. The visibility or 

 invisibility of certain rays depends on the construction of the eye 

 rather than on any peculiarity of the rays (649&). It is quite 

 possible that the eyes of some animals are so constructed that ultra- 

 red rays may excite vision, and that the eyes of other animals are so 

 constructed that ultra violet rays may excite vision. 



653. The Electric Light. The electric light is particu- 

 larly rich in. these invisible rays. The dark heat rays may be sifted 

 from the beam of light by passing it through a transparent solution 

 of alum ; only the luminous rays will be allowed to pass. The 

 luminous rays may be sifted out by sending the beam through an 

 opaque solution of iodine in carbon bi-sulphide. If these solutions 



