86 Unit of Candle-power in White Light. 



unusual to find that a lamp has appeared to change during 

 12 months by more than O'l per cent., and the majority of 

 them show no change which can be detected. If any lamp 

 in a set shows a difference from the mean of 0*1 percent, no 

 change in the value assigned to it is made until such difference 

 is repeated in the following year's analysis. Of the one or 

 two lamps whose values have had to be adjusted in this way,, 

 some appear to have risen and some to have fallen in candle- 

 power, and there is no sign that any fundamental set of 

 lamps is undergoing a progressive change. This can be said 

 equally of the tungsten filament lamps as of those with 

 carbon filaments. A set of 1/5 watts per candle tungsten 

 standards (set 6) has to be used nearly every day of the 

 week, and a fundamental as well as a working set of these 

 lamps is therefore kept. The working set fell in candle- 

 power about 0*5 per cent, during the continuous use of the 

 past two years. As the fundamental set will only be used 

 perhaps two or three times a year, its constancy for very 

 many years is assured. It could at any time be compared 

 against set 5 (2 watts per candle), which has never to be- 

 used in ordinary routine work, and which, as a matter of 

 fact, has not been used since the values of its individual 

 lamps were first fixed by the cascade method. Similarly, 

 the use of set 1 (matching the pentane lamp in colour) will 

 never be necessary in ordinary work because of the redness 

 of the light. When it is remembered that the filaments of 

 these lamps are of tungsten operating at the low efficiency 

 of about 7 watts per candle, and that they only require to- 

 be used for exceptional reference purposes at intervals of 

 several years, there would seem to be every reason for 

 expecting the unit of candle-power to be maintained constant 

 by means of them for an indefinite period. 



The authors desire to place on record their obligations to 

 Dr. R. T. Glazebrook, C.B., F.R.S., Director of the National 

 Physical Laboratory, and on behalf of the Laboratory to 

 acknowledge the generosity and help of the General Electric 

 Co. and the Osram Lamp Works for the many expensive and 

 special standard lamps which they have made and presented 

 in connexion with this work. 



