246 
NATURE 
| Fan. 26, 1871 

Yellow 
‘ In the paper on colour read by Mr. Strutt before the British 
Association (NATURE, Jan. 19) two things are stated as requiring 
explanation (p. 238), both of which, it seems to me, are explained 
by one of the results furnished by such experiments as his own. 
The first is the difficulty usually found in recognising the 
demonstrated fact that yellow is a compound colour. The other 
is that we generally distinguish different kinds of yellow more 
strictly than of other colours. ‘‘ A dark yellow or orange 
3 suggests its colour so little as to be called by a new name 
(brown), while a dark blue is blue still ;” upon which I must 
observe that we thus distinguish degrees of impurity rather than 
degrees of darkness, and that an impure yellow is called brown 
when it is dark, and drab when it is light. , 
Both things, however, are explained if it is true that natural 
yellows differ less from the nearest colours of the spectrum than other 
natural colours do. For, in the first place, the consequence will 
be that there will be many yellows in nature which we could not 
compound by our ordinary reds and greens, and we therefore find it 
difficult to imagine it could be compounded by any red and green. 
Secondly, there will be a greater relative range, so to speak, of 
yellowness, which we shall naturally subdivide according to de- 
grees of purity and brightness. It may be added that, so far as 
brightness is concerned, the greater maximum brightness of yellow 
would act in the way last described. 
But is it true that our yellows differ less from the nearest colours 
of the spectrum than our other colours do? I certainly think 
this is the result of the experiments, but I will only show that it 
is the case with the pigments employed by Mr. Strutt. He says, 
indeed, that ‘‘the most saturated yellow can be compounded of 
red and green.” So it may in the spectrum (Maxwell, P/z/- 
Trans., 1860, p.57—84); but Mr. Strutt’s yellow cannot be com- 
pounded of Mr. Strutt’s red and green. On the contrary, we see 
from his last ‘‘ calculated” equation but one, that his yellow had 
to be diluted with nearly two-thirds as much of his white (the 
brighter of the two, I presume) before it could be matched by 
his red and green. 
We can test the matter more closely. Denoting Mr, Strutt’s 
red, green, and blue by 7, g, 4, and the “primary” red, green, 
and blue of the spectrum by &, G, B (Maxwell, ui supra, p. 74), 
we may thus express the former in terms of the latter :— 
r=LIR+mCG+nB 
g=lR+MG+2B - (1); 
PGR ee vB 
where we know thus much about Z, m, &c. ; first that they are 
all positive, or else extremely small ; secondly that, among them, 
the large letters must denote comparatively large quantities, and 
the small letters small quantities. Now, by the second ‘‘ calcu- 
lated” equation of the second batch, we find that Mr. Strutt’s 
yellow is 
gy (ie84r + 63°5¢ - 494); 
and, if we substitute in this expression the expressions given 
above (1) for 7, g, 4, we shall have, for the coefficient of B, 
= (128'4 2' + 63°5 2 — 49 A) 
Now this must not be negative, or else must be very small in 
comparison with the coefficients of R and G (Maxwell, 7di supra, 
Tables VI. and IX.). Therefore V cannot be greater than 
2°6 n' + 1°3 2; that is to say, there is no more “ primary”’ 
blue in Mr. Strutt’s blue than about 24 as much as in his red, 
plus 1% as much as in his green, It is true that blue is rather a 
dark colour in pigments ; but so it is in the spectrum; and VV 
measures, not a quantity of colour simply, but its ratio to the 
‘‘primary” blue of the spectrum. LEither the yellow was very 
pure, or the red and green very impure ; and, if Mr. Strutt 
provided himself with good representatives of natural colours, 
this proves my point. C. J. Monro 
Jan. 20 
The Primary Colours 
I HAVE been greatly interested in reading Mr. Strutt’s curious 
experiments on colour in the last number of Nature. I am 
glad to see that he is able to assume as proved the theory that 
green and not yellow is the middle primary. The true position 
of green is well illustrated in Mr. W. Benson’s “ Principles of 
the Science of Colour,” both by argument and by diagrams, 


There is, however, one piece of evidence which seems to me 
conclusive as against yellow, but which I have not seen noticed. 
When a solid body is gradually heated to incandescence, the 
light given out is first ved, then orange, afterwards yellow, and 
finally whzte. If yellow were a primary, it would be impossible 
for it to appear in this series, which is formed upon the basis of 
the first primary, red, by successive additions of more and more 
rapid vibrations. Every colour in the series except the first 
must be a compound. If the heat is not sufficient to generate 
the most rapid vibrations which the eye can appreciate, white 
light is not given off at all, the series terminates with the yellow, 
The light of a glowing coal, without flame, in an ordinary fire, 
rarely passes beyond the yellow stage, and such light yields to 
the prism abundance of red and green rays, but scarcely a trace 
of blue or violet. 
But, if red is the first primary, and green the second, which is 
the third? Shall blue still sit upon the throne on which Newton 
placed him, when his brother yellow is deposed? I think his 
position has become extremely precarious, and that he would be 
wise to abdicate with dignity before he is ignominiously turned 
out as a usurper. 
If the kingdom of light is really divided into three princi- 
palities, is not violet the rightful heir to the third throne? Violet 
is said to be a mixture of blue and red. But how should red 
make its appearance at the wrong end of the spectrum? If 
it has no definite limit, but gradually thins out from its own 
place to the other extremity of the spectrum, then the whole of 
the other colours must be more or less affected by it, and red 
must be the only true primary among them. If it is said that 
the red in violet is clearly recognised by the eye, I think it may 
be answered that this is only because we have been taught to 
think of it as a compound, and that we might just as truly say 
that we can see yellow in green or orange in red. 
Leicester, Jan. 21 FREDK. T. Morr 

Utilisation of Sewage 
WHILE heartily thanking your reviewer for the very valuable 
suggestions which he has given me with regard to the second 
edition of my ‘‘ Digest of Facts relating to the Treatment and 
Utilisation of Sewage,” I wish to point outa slight oversight 
which he has made. 
He says, ‘‘Mr. Menzies’ name, however, has somehow or 
other slipped out of his pages (159, 169) where he treats of this 
improvement on the older plans for sewage.” 
The fact is that page 145 has somehow or other escaped the 
reviewer's critical eye. On that page the following sentence 
occurs :—‘* Some of these towns, then, it will be seen, are pro- 
vided with sewers much upon the plan which Mr. Menzies has 
the credit of having first brought prominently forward ; that is to 
say, with impervious pipes for the sewage properly so called, and 
brick drains for the surface and storm water, the former being 
laid deeply and the latter being superficial.” 
I should be very sorry to have it thought that I had forgotten 
‘to emphasise the name of the Windsor Sanitarian.” 
W. H. CorFIELD 

Ocean’ Currents 
THE differences of barometric pressure to which Mr. Keith 
Johnston refers (NATURE, Jan. 19, p. 227) have a well-ascer- 
tained geographical existence, but his suggestion that they may 
originate or direct the Ocean Currents is clearly inadmissible. 
The high pressure over a large patch of the North Atlantic to 
the south or south-west of the Azores—and similarly in each of 
the other oceanic basins—is there permanently ; and whatever 
disturbance might be produced by it was produced once for all 
when the high pressure was first formed. It would then displace 
a certain quantity of the water over which it rested, would 
thrust it out, and keep that particular part of the ocean at a 
slightly lower level than that over which the pressure of the air 
was not so great. But having done this, the adjacent bodies of 
water would be in hydrostatic equilibrium, and the high pressure 
could not continue to thrust water out towards the place of low 
pressure. My meaning may be at once illustrated by putting one 
end of an open glass tube into a basin of water, and partially 
exhausting the air inside it. The adjacent surface is thus ex- 
posed to a higher pressure than the surface inside the tube, and 
a certain portion of the water is immediately thrust from the 
place of greater to the place of less pressure ; the column of 
water inside the tube is raised until the weight of the excess 
