SPECTRUM. 



315 



visible rays and the less refrangible so-called "heat rays" can, 

 however, produce some degree of chemical action; whilst the 

 visible rays and the ultra-violet ones are by no means destitute of 

 heating power ; so that the division into heat rays, visible light, 

 and actinic rays, although convenient, is not absolutely exact. 



Let a room fronting the sun be closed by a shutter in which a 

 small hole is perforated, so that a beam of sunlight can be admitted 

 through the orifice. Hold a glass prism base upwards in the track 

 of the beam of light (fig. 157), and at some feet (or even yards) 

 distant hold vertically a sheet of white cardboard or other con- 

 venient white flat surface, so as to receive the refracted rays. 



It will now be noticed that, instead of a white spot of light 

 being produced, as would be the case were the white screen held 

 in the beam of light before it meets the prism, a band of colours 

 or spectrum will be developed in the order above enumerated. 



Fig. 157. Spectrum. 



The lower end of the band will be red, being due to the least 

 refrangible of the visible rays ; whilst the upper end, obviously 

 that due to the rays most deflected from the original path, will be 

 violet. By holding the bulb of a thermometer in the different 

 colours successively, and in the spaces just beyond the red and 

 violet ends of the spectrum respectively, it can be shown that the 

 greatest heating effect is produced by the invisible rays of slightly 

 less refrangibility than the red ; whilst by similarly holding a test- 

 tube containing a solution of sulphate of quinine in the same 

 different positions, the production of a pale blue light emitted from 

 the liquid by fluorescence (Pa. 290) will be seen to take place to 

 the greatest extent by the agency of the invisible ultra-violet rays. 

 Similarly, photographic paper (e.g., that prepared with silver 

 chloride, Expt. 392) will be most acted upon by these same ultra- 

 violet rays, and hardly at all, even after long exposure, by the less 



