542 SCIENCE PROGRESS 



barrassing in the manufacture of metallic-filament lamps is a 

 sacrifice which we have to pay for their abnormally high 

 luminous efficiency. 



It seems probable that it would be difficult to secure any- 

 thing approaching the theoretically ideal luminous efficiency 

 by means of an incandescent solid. This, at least, is what our 

 present experience suggests. Yet, as mentioned previously, 

 the fact that solids afford a continuous spectrum is a consider- 

 able advantage from a practical standpoint, as the light allows 

 reasonably good colour definition. It is even possible, by 

 means of a special screen, to convert the light from a tungsten 

 lamp into a close approximation to daylight. This appears to 

 have been effected recently in the United States by Dr. H. E. 

 Ives — naturally at the expense of a certain sacrifice in effi- 

 ciency. Lamps provided with suitable screens to give the 

 daylight effect are stated to have a specific consumption of 

 about 10 watts per candle ; the high consumption of power, 

 however, is often of small consequence in comparison with the 

 advantage of gaining an "artificial daylight" enabling delicate 

 colour-matching work to proceed by night. Similar screens 

 have also been devised for use with enclosed arc lamps and 

 Mr. T. E. Ritchie, in a recent paper before the Illuminating 

 Engineering Society, has pointed out that the resemblance of the 

 light from an inverted open arc lamp to daylight is very much 

 closer than is generally supposed — indeed, he thinks, probably 

 closer than that of any other illuminant commonly used in this 

 country. At this meeting a tungsten lamp provided with a 

 special daylight screen was also shown for the first time by 

 Dr. Kenneth Mees and it was stated that the resemblance 

 to natural daylight is exceedingly close. 



Assuming that we can specify what we mean by white light 

 sufficiently closely for practical purposes, meaning thereby the 

 colour of normal diffused daylight, it is of interest to ask what 

 the ideal efficiency would be of a source of light producing 

 these particular rays in the desired proportions and no others. 

 The " mechanical equivalent" of white light has been studied by 

 a number of observers and the results have been subjected 

 frequently to revision. It is not possible, of course, to deduce a 

 factor for converting energy into light in the same rigid manner 

 as can be done for heat energy. We can, however, estimate 

 what would be the output in candles per watt of our ideal 



