314 SCIENTIFIC AMUSEMENTS. 



the base of the prism, in accordance with the law of sines (Expt. 

 333) ; on emerging it is again refracted, but not into a line parallel 

 with ab, the path taken being along cd ; so that an observer whose 

 eye catches the ray, perceives an image of the object from which 

 the ray was emitted at e, the object being apparently deflected 

 towards the edge of the prism, the extent of the deflection varying 

 with the refracting angle of the prism, i.e., the angle between 

 the two inclined plane surfaces. 



Expt. 349. Production of a Spectrum. In the previous 

 experiment, if the object viewed be illuminated with one kind of 

 light only (e.g., that emitted from the flame of spirits of wine and 

 salt, Expt. 245), all that will be noticeable on viewing it through 

 the prism will be displacement, as stated ; but if sunlight or any 

 other white light be employed, the image will be fringed with 

 colours. This arises from the fact that differently coloured lights 

 are differently refracted by one and the same medium, or more 

 exactly, that radiant energy generally consists of emanations of 

 different degrees of refrangibility ; that is, bent out of the original 

 path to different extents by one and the same refracting medium. 

 Those rays that are least refracted are invisible to the human eye, 

 but possess to a large extent heating qualities, and are accordingly 

 often spoken of as invisible heat rays or radiant heat (Chapter 

 XXIV.) ; of those rays that are visible to the eye, the least refran- 

 gible produce the sensation termed red colour, and those which are 

 most refrangible the sensation termed violet colour, intermediate 

 between which lie a number of other colours corresponding with 

 intermediate degrees of refrangibility, the order being : * 



Red, . . . Least refrangible. 



Orange, . . \ 



Yellow, 



Green, . . ^Intermediate. 



Blue, ... 



Indigo, . . / 



Violet, . . Most refrangible. 



Beyond the violet rays come a series of others more refrangible 

 still, but, like the heat rays, invisible to the human eye (not 

 necessarily so to other animals) ; these rays possess, to the greatest 

 extent of any, the power of setting up chemical action (Chapter 

 XXV.), and of exciting fluorescence (supra), and are therefore 

 generally designated the actinic or ultra-violet rays. Both the 



* Taking the colours in the reverse direction, the order of succession may 

 be conveniently remembered by means of the nonsense-word vibgyor, com- 

 posed of the initial letters of the names of the tints. 



