1914-15.] Reflective Power of Pigments in the Ultraviolet. 151 
With cardboard used as a standard a considerable increase in the values 
obtained for most of the pigments takes place towards the extreme ultra- 
violet end of the iron arc spectrum. As this might simply be due to a 
falling off of the absolute reflective power of cardboard, it was thought 
advisable to compare the latter with ground quartz in the same manner. 
The quartz was backed by black velvet, which was found to reflect 
practically no light diffusely from the spectrum, in order to prevent 
reflection at the second surface, and gave the following results : — 
Wave-length in Micromillimetres. 
445. 
388. 
322. ^ 
274. 
240. 
White cardboard 
1-00 
100 
1-00 
]-00 
100 
Ground quartz ..... 
•09 
•10 
•12 
•13 
•13 
Quartz exerts no selective reflection in this region of the spectrum, so 
that its reflective power will vary uniformly throughout, and will probably 
be somewhat greater the shorter the wave-length. It is likely, therefore, 
that the white cardboard has a more or less constant reflective power at all 
the points examined. Even if this were not so, the decrease in reflective 
power of the cardboard in the extreme ultraviolet as compared with quartz 
is not of itself sufficiently great to account wholly for the increase observed 
in the case of many of the pigments. There is thus evidence of selective 
reflection on the part of the latter in that region. Further observation 
would be valuable in the extreme ultraviolet with a more extended 
spectrum — for example, Nutting’s spark spectrum, which extended to 
the aluminium line of wave-length 1850 A.U. 
Messrs Winsor and Newton, Ltd., have kindly supplied me with notes 
on the chemical composition of the pigments. These bring out some 
striking anomalies. Flake white, though similar chemically to Cremnitz 
white and silver white, shows a drop in reflective power as compared with 
them in the middle of the ultraviolet. Chinese white, a dense variety of 
zinc oxide used only in water colour, is an exception to the rule that the 
relative reflecting power increases towards the extreme ultraviolet, while 
zinc white, which is also composed of zinc oxide, obeys that rule. And 
though the essential constituent of light red, Indian red, and Venetian red 
is in each case sesquioxide of iron, the first of these pigments has a lower 
reflecting power throughout the ultraviolet than the other two, whose 
values are very similar. 
The black pigments examined, which are carbon blacks obtained by 
different processes, reflect a considerable proportion of ultraviolet light. 
