THE LUMINOUS EFFICIENCY OF ILLUMINANTS 553 



violet rays present. Careful experiments have established the 

 destructive action of these rays on bacteria. 



In conclusion we may summarise the contents of this article 

 by pointing out that in practice there are many special problems 

 in illumination, each demanding not only a certain intensity but 

 a certain quality of light. We are still far from the ideal of 

 controlling our illuminants so perfectly as to generate just the 

 variety of light we require and no other for a specific problem. 

 Yet progress in this direction is being made. We may even 

 picture the day when, by some easy adjustment, we shall be able 

 to alter the light from a source at will and cause it to give 

 out successively rays from any desired part of the spectrum — 

 to play upon the visible gamut of light with the same ease and 

 certainty as upon the audible octave. 



