418 CHROMATICS SPECTRA. 



ous parts of the spectrum it will be found that the tem- 

 perature is scarcely affected in the violet, but that there is 

 a continual increase in temperature as the thermometer is 

 moved toward the other end of the spectrum, it being quite 

 marked in the red. The greatest augmentation of tem- 

 perature takes place beyond the red, wholly outside the 

 visible spectrum. We thus detect ultra-red rays con- 

 stituting a heat spectrum. Their position indicates 

 their low refrangibility and increased wave-length. Be- 

 cause of its diathermancy, a rock-salt prism is desirable for 

 this experiment; glass absorbs most of the ultra-red rays. 



651. Actinic Spectrum. The actinic or chemical 

 effects of sunlight are, in a general way, familiar to all. 

 For example, plants absorb carbon from the atmosphere 

 only during the day time. Silver chloride is very sen- 

 sitive to this action of sunlight. The sensitive paper 

 of the photographer will remain unchanged in the 

 dark ; it will be quickly blackened in the light. If a 

 piece of paper freshly washed in a solution of sulphate of 

 quinine, or some other fluorescent substance, be held in the 

 ultra-violet rays, it will become visible. Such a slip of 

 paper may be used as a test for the presence of actinic rays. 

 By placing it successively in the different parts of the visi- 

 ble spectrum, it will be affected least in the red and most 

 in the violet. The maximum actinic effect will be found 

 at a point beyond the violet, wholly outside the visible 

 spectrum. We thus detect ultra-violet rays consti- 

 tuting an actinic spectrum. Their position indicates 

 their high refrangibility; that their wave-length is less 

 than that of the violet rays. A quartz prism is desirable for 

 this experiment as glass quenches most of the actinic rays. 



