1876.] 
The Constants of Colony , 
461 
surfaces, which we find reflect in both cases eight-tenths red 
light and two-tenths white light. In spite of this the tints 
may not match, one of them being much brighter than the 
other, containing, say, twice as much red light and twice as 
much white light ; having, in other words, twice as great 
brightness or luminosity. The only mode of causing the 
tints to match will be to expose the darker-coloured surface 
to a stronger light, or the brighter surface to one that is 
feebler. It is evident then, that brightness or luminosity is 
one of the properties by which we can define colour ; it is 
our second colour constant. This word luminosity is also 
often used by artists in an entirely different sense, they 
calling colour in a painting luminous, simply because it 
recalls to the mind the impression of light, not because it 
actually reflects much light to the eye. The term “ bright 
colour” is sometimes used in a somewhat analogous sense, 
but the ideas are so totally different that there is little risk 
of confusion. 
The practical determination of the second constant is 
possible in a great many cases ; it presents itself always in 
the shape of a rather troublesome photometric problem, 
capable of a more or less accurate solution. The relative 
brightness of the colours of the solar spedtrum is one of the 
most interesting of these problems, as its solution would 
serve to give some idea of the relative brightness of the 
colours which, taken together, constitute white light. 
Quite recently a set of measurements were made in different 
regions of the spedtrum by Vierordt, who denoted the points 
measured by the fixed lines, as is usual in such studies.* 
The following table will serve to give an idea of his results : — 
Colour. 
Degree of Luminosity. 
Dark red ..... 
Red 
. . . 4,930 
Red, slightly orange . . 
. . . 11,000 
Orange -re cl 
• • • 27,730 
Orange 
. . . 69,850 
Yellow 
Green ...... 
Cyan blue ..... 
Blue 
. . . 4,930 
Ultramarine blue . . . 
Violet 
• • ' 359 

... 131 
,, ...... 
... 58 
,, ...... 
... 9 
* C. Vierordt, Poggendorff ’s Annalen, Band cxxxvii., S, 200, 
