426 REPORT—1883. 
rods had failed hitherto to arrive at any consistent results, and it would be neces- 
sary under these circumstances that such dependence should be established upon 
a firm basis before it could be admitted as an accomplished fact in photometry. 
11. On a Lamp giving a Constant Light. 
By A. Vernon Harcourt, JA., F.R.S. 
Six years ago the need of a standard light for photometry, and a proposal to obtain 
such a light from a burner of simple construction, consuming a definite mixture of 
air and petroleum vapour, were brought before this Section. The composition of the 
‘air-gas’ and the height of the flame were so adjusted that the light, which was 
easily kept constant, was equal to the average light of one of the sperm candles 
made for photometry. Two years later, the attention of the Board of Trade 
having been called to the uncertainty as to the quality, or price, of gas supplied 
by the gas companies, arising from the variable character of the standard 
candles, a committee, consisting of Dr. Williamson, Dr. Odling, and Mr. G, 
Livesey, was appointed to inquire into existing and proposed photometric 
standards. After a prolonged inquiry, the details of which are printed in a 
Report to the Board of Trade, the committee pronounced sperm candles to 
