394 Mr. L. Schwendler on a 



photometer, and consequently its quantitative effect on a given 

 point. These difficulties might be overcome to a certain ex- 

 tent by mechanical means — as, for instance, by cutting the 

 wick automatically within equal and short intervals of time, 

 and by placing the candle in a closely fitting metal tube, 

 against the top rim of which a spring presses the burning 

 candle — in fact, a similar construction to that used for carriage- 

 candles. But, to say the least, all such arrangements are 

 cumbersome. Without going into further details with refe- 

 rence to the Carcel burner, it may be said that the disadvan- 

 tages of this standard are at least equally great. In fact it 

 appeared to me that the production of a standard light by 

 combustion is not the right method ; the flame resembles too 

 much organic life with its complex and incessantly varying 

 nature. Gauging mechanical force by the power a particular 

 horse of a certain breed is able to exert, can scarcely be called 

 a less scientific standard than the combustion standard for 

 measuring light. Under these circumstances I thought it 

 best to leave the old track, and produce the standard of light 

 by the heating effect a constant current has in passing through 

 a conductor of given mass and dimensions*. 



New Standard of Light. — Several platinum photometric 

 standards were made and tried. If the current passing through 

 the platinum was kept constant, the light produced was also 

 constant; and for the same current and the same platinum 

 standard the light was always of the same intensity, under 

 whatever other circumstances the experiments were con- 

 ducted. 



Platinum evidently is the best metal which can be chosen ; 

 for it does not change in contact with oxygen, it can be pro- 

 cured very pure, and its melting-point is high enough to allow 

 an intense light. 



It is probable that at a high temperature platinum becomes 

 volatilized ; but this process can only be exceedingly slow, and 



* The idea of using the light produced by a conductor through which 

 a strong current passes as the unit of light appeared to me so natural and 

 simple, that I could scarcely understand why it had not been proposed 

 and acted upon before. 



I could, however, find nothing on the subject anywhere, until lately 

 my attention was called to a small pamphlet written by Zollner in 1859, 

 in which the same idea occurs. In the preface to his Inaugural Disserta- 

 tion, Zollner says : — " Andererseits aber auch zu zeigen, dass ein garvanisch 

 gliihender Platindraht von den bis jetzt bekannten Lichtquellen zur Auf- 

 stellung einer photometrischen Einheit, trotz mancher practischer Schwie- 

 rigkeiten, yielleicht dennoch das geeignetste Mittel sei." 



I have since learnt that Dr. Draper, as early as 1844, proposed a "unit 

 lamp " consisting of a platinum strip heated by an electric current. 



