ON COLOUR PHOTOMETRY. 
451 
carve of the arc crater, in fact, this former curve more nearly resembles that of sun¬ 
light. Now we have already shown in previous papers that the solar spectrum gives 
evidence of the light having passed through a considerable quantity of water “ stuff,” 
and if the sunlight before it reaches our atmosphere has anything in common with 
the light emitted by the carbon, the modification of the latter in passing through the 
trough of water would be a further confirmation of our view. We hope to enter into 
this subject more fully in a subsequent communication, but we may say, in passing, 
that the observations which we have made of the diminution of the light in different 
parts of the spectrum, after passing through still greater thicknesses of water, are of 
great interest, and accord well with the results of corresponding observations made 
with the thermopile. 
§ XXII. Curves of Luminosity of Carbon at Different Temperatures. 
It was not only a matter of curiosity, but during our investigations it became a 
matter of necessity, to examine into the illuminating values of incandescence lamps at 
different temperatures. Although the full results of our experiments in this direction 
are scarcely germane to this paper, we think it well briefly to notice some of them. 
Fig. 10, Plate 25, gives the curve of luminosity of a lamp with a carbon filament in a 
vacuum of ^-millionth of an atmosphere after all gases had been exhausted, both from 
the filament and from the glass globe enclosing it. A secondary battery supplied the 
current, and it may be assumed that the temperature remained constant during the 
whole of each experiment. 
Some of the measurements were made by the method of using a patch of mono¬ 
chromatic light, others by that of throwing on the screen the whole spectrum, which 
measured from A to H 18 inches. 
The latter, as already pointed out, is not one which lends itself to accuracy unless 
precautions are taken; on the other hand, the greater brightness of the spectrum on 
the screen renders it preferable to the patch method for feeble lights. From a careful 
comparison of the observations made with the same source of light by the two methods, 
a constant was determined for converting the results of one method into the terms of 
the other. 
During these experiments no part of the apparatus was even touched ; everything 
remained unchanged, except the amount of current, and therefore the temperature of 
the filament. With the current of approximately 2^ amperes from 26 boxes of a 
secondary battery, and an E.M.F. of about 491? volts, the filament was of a fairly 
bright orange colour, and its curve of luminosity is given as C, fig. 10, Plate 25. 
Curve B represents the result of passing the current from 28 boxes through the lamp. 
They gave a current of 2§ amperes and an E.M.F. of 53 volts; curve A, that from 34 
boxes, which approximately gave amperes and 64^ volts. 
The light represented by curve B appeared to the naked eye a very bright orange, 
and would certainly be estimated to be as white as gaslight; the lamp would be 
3 m 2 
