DISCOVERY 



159 



its physiological effects. Medical heat-baths, in which 

 the patient stands in a cabinet surrounded by a mass 

 of incandescent lamps, have been devised, and the 

 ordinary electric radiator emits almost exclusively 

 rays of this nature. Common experience suggests 

 that the infra-red radiation present in most artificial 

 illuminants has no material prejudicial effect on 

 vision, provided the sources are not brought too near 

 the eyes and- are used discreetly. But continual 

 (_ xposure of the eyes to glowing masses at close quarters 

 is doubtless bad, and the cataract that is somewhat 

 prevalent amongst glassworkers and others who work 

 in close proximity to incandescent material has been 

 ascribed to this cause. 



Much farther down the scale we come to the electro- 

 magnetic waves used in wireless telegraphy and 

 telephony, and between there exists a wide range of 

 infra-red radiations of whose properties little is 

 definitely known. 



human body furnishes an appreciable amount of heat, 

 instruments which can measure very small quantities 

 of heat could be used to detect the approach of an 

 enemy at night. It is stated that by this device men 

 were easily observed at a distance of 6co feet, and that 

 a person lying on the ground 400 feet away was 

 detected unerringly as soon as he lifted his head above 

 the ground. Thus enemies creeping towards the 

 trenches in the dark could be noted and a warning 

 given. The method is also stated to have found some 

 use in detecting the approach of hostile aircraft, and 

 also as a basis of secret signalling. Experiments on 

 the use of rays at the other extreme end of the spectrum, 

 the ultra-violet, for secret signalling have also been 

 made by the U.S..\. Army, and no doubt much work 

 of a similar nature was also done for the British 

 forces. 



The most striking effects of ultra-violet light are 

 met with in mountainous regions. These rays are 



V\tra -Violet 



LIMIT5 OF VISIBLE SPECTRUH ), 



"1 r 



0002 OOOOi 0004- 0005 0006 0007 0008 0O09 0-001 



Fig. I.— showing ROUGHI^Y the DISTRIBUTIGX OF HEAT, LIGHT, AND CHEIIICAL (PHOTOGRAPHIC) EFFECT THROUGHOUT 



THE SPECTRUM. 



The three effects of radiation in the visible and 

 adjacent parts of the spectrum with which we are most 

 familiar are heat, light, and photographic effect. 

 These are distributed roughly in the manner indicated 

 in Fig. I. Interest centres chiefly on the invisible 

 rays of wave length shorter than the extreme violet — 

 the " ultra-violet " which plays a considerable part in 

 promoting chemical action as illustrated by the effect 

 on the ordinary photographic plate. 



Apart from their use as a curative agent, or for 

 heating dwellings, infra-red rays adjacent to the 

 spectrum have not as yet found many practical 

 applications. But a highty interesting application of 

 such rays occurred during the war. It occurred to 

 several investigators connected with the U.S..\. 

 Trench Warfare Research Division ' that, as the 

 ' Physical Review, August 1919. 



rapidly absorbed by the earth's atmosphere — so much 

 so that experimenters with rays of very short wave- 

 length have found it necessary to cause them to pass 

 their instrument through an evacuated glass tube. 

 In the rarefied atmosphere at high altitudes the effect 

 of these rays, accentuated by reflection from the 

 snow, is very evident, and mountaineers now habitually 

 use goggles to protect their eyes, and a mixture of 

 lanoline and pigment to cover their skin. The influence 

 of altitude is well shown in the accompanying diagram, 

 based on Professor S. P. Langley's observations at 

 the base and the summit of Moimt Whitney in the 

 Sierra Nevadas, 15,000 feet high. 



Light Sources and Ultra-violet Light 



Although the great majority of artificial sources of 

 light, which depend on incandescence, furnish little 



