150 
Proceedings of the Royal Society of Edinburgh. [Sess. 
placed in turn over the less dense of two images under observation until 
one was found which made the images appear of equal density. The 
fraction of light transmitted by the strip gave the relative density of the 
two images. After a time the visual estimations were found sufficiently 
reliable, and the use of the comparison strips was dispensed with. 
Care has to be observed in making photometric comparisons by means 
of photography. Strictly speaking, the relative densities of the images 
produced by independent sources is an accurate measure of the relative 
intensities of the sources only when images of equal density are produced 
simultaneously on adjacent portions of the same plate by sources of the 
same wave-length. The numbers tabulated below can therefore be regarded 
as approximations only, and are intended to indicate the order of magni- 
tude rather than the actual values of the respective relative reflecting 
powers. 
Relative Reflective Powers for Ultraviolet Light. 
Wave-length in Micromillimetres. 
445. 
388. 
322. 
274. 
240. 
White cardboard 
1-00 
LOO 
LOO 
LOO 
LOO 
Flake white 
LOO 
•70 
•20 
•81 
L40 
Cremnitz white ..... 
103 
LOO 
L08 
1-23 
2 03 
Silver white 
1-00 
LOO 
1-00 
1-05 
1-15 
Zinc white ...... 
LOO 
•99 
•89 
L20 
L85 
Chinese white 
1-00 
•93 
•34 
•14 
•02 
Pale chrome yellow .... 
•36 
•38 
•35 
■84 
L16 
Naples yellow, French ... 
•70 
•80 
•78 
LIO 
L40 
Yellow lake ...... 
•00 
•11 
•09 
•22 
2-48 
Yellow ochre ...... 
•26 
•33 
•21 
•88 
L13 
Transparent gold ochre .... 
•36 
•45 
1-18 
1-20 
2-45 
Crimson lake 
•00 
•01 
•02 
•05 
•10 
Yermilion 
•18 
•32 
•16 
•99 
•99 
Light red ...... 
•31 
•36 
•34 
•58 
•82 
Indian red ...... 
•50 
•53 
•72 
1-00 
L30 
Venetian red ...... 
•41 
•50 
•70 
1-23 
L25 
Chrome green, No. 3 
•16 
•39 
•58 
1-00 
3-93 
Cinnabar green (deep) .... 
•29 
•36 
•39 
•99 
2-43 
Emerald green 
•15 
•25 
•13 
•86 
LIO 
Cobalt blue 
•02 
•01 
•01 
•20 
•68 
Prussian blue 
•00 
•00 
•00 
■00 
•00 
Antwerp blue ...... 
•04 
•11 
•02 
•18 
100 
Indigo ...... 
•18 
•26 
•26 
•93 
3-33 
Burnt sienna ...... 
•20 
•30 
•20 
•80 
115 
Vandyke brown ..... 
•58 
•68 
•73 
•85 
LOO 
Brown ochre 
•45 
•58 
•69 
LIO 
4-23 
Raw umber ...... 
•29 
•38 
•50 
1-28 
3-38 
Bone brown ...... 
•49 
•53 
•67 
LOO 
L13 
Lamp black ...... 
•44 
•50 
•56 
LOO 
L98 
Ivory black . 
•27 
•51 
•62 
•84 
L78 
Blue black ...... 
•45 
•53 
•70 
LOO 
L33 
Payne’s gray ...... 
•13 
•26 
•20 
•56 
•94 
Charcoal gray 
•31 
•38 
•38 
•51 
•58 
Davy’s gray 
•40 
•48 
! 
•53 
1 
•86 
•95 
