Synopsis.1. The differences in the colour of the light radiated from different sources of white light causes uncertainties in photometric determinations. The Paper describes the methods used a t the National Physical Laboratory to minimise this wellknown difficulty by the use of the cascade principle.2. The six sets of electric substandards of candle-power are described, which vary in the colour of the light radiated, from that of the pentane lamp (red) to that of a tungsten vacuum lamp operating a t 1.5 watts per candle.3. The differences between the values obtained by different observers in the process of stepping from one colour t o the next are discussed and compared with those obtained by the direct comparison of the first and last sets of substandards.
The probable errors of the determination are discussed ahdshown to be of the order of 0.2, per cent. 5. The :rbsolcite value of the unit of candle-power has been re-determined from the pentane lamp, and found to agree with the determination made by one of the authors 10 years ago to within less than 0.1 per cent.6. The corrections to the pentane lamp for humidity and barometric changes were also redetermined, and found to agree within narrow limits with those previously found by one of the authors.
7.The relatim between the humidity and temperature corrections for the pentane lamp is discussed, and the fact that these two effects may act together is suggested as the reason for a discrepancy which has been noticed between the humidity constants of the lamp as determined in London and Washington.* The work dealt with in the various sections of this Papor has entailed SO many thousands of measurements and candle-power determinations of individual lamps, that to give them in detail would greatly overweight the Paper with tabular matter. The authors have, therefore, felt obliged to limit themselves to summaries of results and sometimes t o bare statements of the results obtained, computed from their manuscript tables. THE UNIT OF CANDLE-POWER, M.S.C.P. M.H.C.P.* Assuming a ratio ofof 0.85 end 0.79 fo; carbon and tungsten filaments respectively, the approximate watts per candle me obtained for any value of lumens per matt by dividing the lattcr into 10.7 for carbon slament lamps, and 10.0 for tungst,en filament lamps.