THE LUMINOUS EFFICIENCY OF ILLUMINANTS 547 



region. In the case of the ordinary mercury vapour lamp, these 

 rays are nearly all absorbed by the glass tube and converted 

 into heat; but more recently lamps have been constructed with 

 tubes of quartz glass, which allows the ultra-violet energy 

 to pass unchecked. This invests such lamps with remarkable 

 properties to which reference will be made shortly. It appears 

 that very little infra-red energy is produced in the mercury 

 spectrum. 



The utilisation of luminescence in the mercury lamp is the 

 explanation of its high luminous efficiency. On the other hand, 

 it is also a striking example of the imperfect control we at 

 present possess over this system of producing light, as the 

 peculiar greenish colour it exhibits unfits the lamp for many 

 practical purposes. The experiences of those who have attempted 

 to modify the spectrum of the mercury lamp are of some interest. 

 Until recently all attempts to supply the missing red element 

 in the light had proved fruitless. One of the first methods tried 

 was to introduce gases into the tube, the idea being that a 

 combined spectrum containing all the primary colours might be 

 obtained by this means. Unfortunately it proved to be difficult 

 to produce both the spectrum of the gas and the mercury vapour 

 simultaneously. For example, when nitrogen was used the 

 colour of the light was either pink or green, according to the 

 temperature. Metallic ingredients, lithium, potassium, etc., 

 were also tried but the results were again untrustworthy and 

 not permanent. 



It appeared therefore that a successful attempt to improve 

 the colour must be made by some means outside the tube. One 

 early device consisted in the combination of the mercury lamp 

 with an outside source, such as a carbon-filament electric 

 incandescent lamp, which is rich in red rays. This, however, 

 proved an inconvenient complexity and naturally entailed a 

 serious sacrifice in efficiency. 



Within the last year, however, an announcement has been 

 made by Dr. Cooper Hewitt of a method which appears much 

 more promising. This consists in placing above the tube a 

 reflector coated with rhodamine and other substances which 

 fluoresce, transforming the blue rays partly into red radiation. 

 As a result, the appearance of the light is much improved. It 

 will be interesting to watch the future of this device. 



This example of the practical utilisation of fluorescence for 



