352 SCIENTIFIC AMUSEMENTS. 



refrangibilities are absent from a spectrum, not only is there less 

 luminous effect at that spot (producing the sensation of compara- 

 tive darkness to the eye) ; but, further, the chemical and heating 

 effects of that part of the spectrum are diminished relatively to the 

 adjacent portions visible to the eye. 



One remarkable point of similarity between visible and invisible 

 light (whether of lowest or highest refrangibility) is that all kinds 

 appear to be propagated with exactly the same velocity ; whilst 

 the same law as to decreasing power as the distance increases is 

 also obeyed in all cases, viz., that the decrease goes on at a rate 

 proportionate to the reciprocal of the square of the distance, so 

 that the light is only one-fourth as intense at double the distance, 

 one-ninth at treble, and so on. 



Expt. 377. To obtain Rays of Low Refrangibility unmixed 

 with Visible Light. This effect is easily obtained by passing a 

 beam of light from a powerful electric arc light, or from the sun 

 (conveniently reflected by a heliostat or mirror kept moving by 

 clockwork, so as always to throw the reflected beam along the same 

 path, no matter how the sun varies in its position in the heavens), 

 through a rectangular trough with glass sides (fig. 193), containing 

 a solution of iodine in carbon bisulphide. If 

 a sufficiently thick layer of this fluid be used, 

 no visible light whatever passes through ; but 

 the invisible heat rays are only slightly ab- 

 sorbed ; hence, if a convex lens be placed in 

 the path of the beam of invisible light exactly 

 the same result will be produced as with 

 visible sunlight, viz., the rays are concentrated 

 to a focus (Expt. 365), so that inflammable 

 objects can be fired and solids heated red 

 hot or even white hot by the accumulated 

 heating effect, just as when the lens is used 

 Fig. 193. Trough as an ordinary "burning glass." The best 

 with Glass Sides. regultg are here obtaine d when the lens is 

 made of clear rock salt, as glass and many other media, though 

 transparent to visible light, largely absorb the heat rays of low 

 refrangibility (Expt. 382) ; this absorption is especially marked 

 with alum (Pa. 292). 



Another way of obtaining a beam of invisible heat rays is to 

 pass the light from a powerful electric arc first through a rock salt 

 lens to concentrate and parallelise it, and then through a rock salt 

 prism or series of prisms ; in this way the total beam is refracted, 

 and the rays of different refrangibility made to traverse different 

 paths mutually inclined to each other ; by interposing dark screens 



