346 

Lecture Experiments on Colour’ 
For some time I have been taking an active interest in the 
phenomena of colour, and have read with much pleasure the 
papers of Mr. Strutt and other gentlemen, and the abstract of the 
interestinz lecture recently delivered at the Royal Institution by 
Prof. Maxwell. I have repeated many of the experiments of 
these observers, and have successfully exhibited them in public 
in a modified form, and in a way which can be readily repeated 
by other lecturers without the aid of the elaborate contrivance 
used by Prof. Maxwell. The following experiments make no 
pretension to rigid accuracy, but are merely described as striking 
lecture-table demonstrations of well-ascertained but little known 
scientific facts. I use the lime light for their exhibition, as it 
suits my convenience better than the electric light, though many 
lecturers prefer the latter. 
A beam of light from the lantern is passed through a slit, 
focussed by a lens, refracted by a disulphide of carbon prism, 
and the spectrum exhibited in the usual way. A flat cell con- 
taining a solution of potassium permanganate is next placed in 
front of the slit. 
of black bands are produced in the green part of the spectrum, 
but with a stronger solution the green and yellow are completely 
cut out, allowing only the red and deep blue lights to pass. On 
widening the slit these bands of coloured light of course increase 
in width also, gradually approaching each other until they over- 
lap, producing a fine purple by their admixture. 
If the experiment be repeated, substituting for the perman- 
ganate an aikaline mixture of litmus and potassium chromate 
in certain proportions, only the red and green light are trans- 
mitted, the blue, and especially the ye//ow, being completely ab- 
sorbed. On widening the slit as before, the red and green bands 
overlap, and produce by their union a very fine compound yellow, 
while the constituent red and green are still visible on each side. 
The effect is most striking when by the widening of the slit a 
round hole is exposed in its place, when there appear on the 
screen two circles, respectively green and red, producing bright 
yellow by their admixture, This experiment is the more striking 
as it immediately follows the process of absorbing the simple 
yellow. The mixture above described (suggested by Mr. Strutt) 
answers better than a solution of chromic chloride. 
Of course, it is a well-known fact that all natural yellows give 
a spectrum of red, yellow, and green, and a common effect illus- 
trating the compound nature of yellow is noticed when exhi- 
biting a continuous spectrum on a screen. When the slit is 
narrow the green is very fully developed, and only separated from 
the red by a very narrow strip of yellow, while on gradually 
increasing the width of theslit the red and green are seen to over- 
lap, producing the brilliant yellow we generally notice. Thus 
the purer the spectrum the less yellow is observed. 
If the continuous spectrum be produced with a quartz prism, a 
little management and adjustment of the distance of the screen 
will cause the two spectra to overlap, so that the red of one may 
be made to coincide with the green, blue, or any desired tint of 
the other. The same result is obtained by employing two slits 
at the same time, the distance between which can be adjusted. By 
this means two spectra are obtained simultaneously, any portions 
of which can be made to coincide. 
I have not tried to use a double refracting Nicol’s prism, as 
is suggested by Mr. Strutt inthe number of NaTurRE for June 22. 
A saturated solution of potassium chromate absorbs all rays 
more refrangible than the green, while a solution of ammonio- 
sulphate of copper stops all éw¢ the blue and green. These 
statements may be proved by placing flat cells containing the liquids 
in front of the slit of the lantern, and on placing one cell in 
front of the other in the same position the green light only is 
transmitted. This experiment serves to explain the reason that 
the mixture of yellow and blue generally results in green, all 
other rays being absorbed by one or other of the constituents. 
By placing the two cells in front of separate lanterns, and 
throwing discs of light on the screen, a beautifully pure white 
is produced where the blue and yellow overlap. 
I employ one lantern only for this experiment, using ‘wo 
focussing lenses side by side to produce the overlapping circles of 
light. 
“also employ a cell with three compartments, containing solu- 
tions of aniline red, ammonio-sulphate of copper, and a mixture 
of potassium chromate with the last solution, and projecting 
images on the screen by means of three lenses fitted on the same 
stand but capable of separate adjustment. I can thus exhibit 
overlapping circles of brilliant red, blue, and green light, which 
NATURE 
With a weak solution and narrow slit a series | 
[Aug. 31, 1871 
produce a perfect white by their admixture, while at the same 
time there is seen the compound yellow produced by the union of 
red and green, the purple arising from the red and blue, and a 
colour varying from grass green to sky blue produced by the 
combination of the green and bluelight. This experiment has 
the advantage of exhibiting at the same time the three primary 
colours—red, grzez, and blue—the compound colours produced 
by their mixture, their complementary tints, and the synthesis of 
white light. 
The flat cells mentioned are made by cutting thin pieces of 
board to the desired shape, and cementing pieces of window glass 
on each side by means of pitch. 
Sheffield, June 26 ALFRED H. ALLEN 
Mr. 
T AM sorry, indeed, that anything ia my answer to Prof. New- 
comb should be unsatisfactory to Mr. Stone. It will certainly 
be hard if after drawing upon myself Prof. Newcom)’s indigna- 
tion by advocating Mr. Scone’s claims, I should find that I have 
unwittingly offended Mr. Stone also. 
It is the misfortune of a weiter on science that he has often to 
deal with overlapping claims; and when he adheres unflinch- 
ingly (as I have always done) to what he regards as the strict 
line of truth, he cuts off a little from the claims on either side, 
and so offends both claimants. I have found myself in the same 
difficulty as respects the work of Dr. De la Rue and Fr. Secc hi, 
in 1860, and I fear the result may have been the same in that 
case also. 
In another case, that of Mr. Lockyer and his fellow-workers 
in spectroscopic solar researches, I freely admit that what I re- 
garded as the line of truth when I wrote *f Tae San,” I now no 
longer regard as strictly such, evidence having been produced 
which has satished me to that effect. Even in this case, how- 
ever, I have in tne first place very little to correct, and in the 
second [am by no means certain that I shall be able to satisfy 
all or any of those coacerned. 
Fortunately or unfortunately, the writer who cannot please all 
proves equally his desire to do justice to all by leaving all dis- 
satisfied. This is commonly the fate of the true neutral. I must 
confess, however, that I cannot see what reason Mr. Stone has for 
being dissatished, since I have ascribed to him, much to Prof. 
Newcomb’s dissatisfaction, the final and complete solution of a pro- 
blem which both have dealt with very ably. Iam still waiting 
to hear the nature of Prof. Newcomb’s objections. Whatever 
they may be, I am assured of /+/s—that in defending (if I can 
defend) my owa work, I shall be advocating Mr. Stone’s claims. 
I hope that in so doing I shall not very grievously offend that 
gentleman, towards whom I eatertain the most friendly feelings, 
RicHarp. A. PRocvoRr 
Stone and Prof. Newcomb 
Saturn’s Rings 
THE reviewer of Lieut. Davies’s work on Meteors has some- 
what misunderstood the extent to which I have been indebted 
(in preparing my treatise on Saturn) to Prof. Maxwell’s excellent 
** Essay on the Saturnian Ring-System.” I have quoted in all two 
and a half lines from that essay, with proper reference to it, and 
I have devoted one-third of a page to summarising the most im- 
portant section of the essay. All the rest of my chapter on the 
Nature of the Rings was written before I had seen Prof. Max- 
well’s contribution to the meteoric theory of the ring-system. 
I may add that every result in Saturn, which is not distinctly 
referred to authority, or else obviously common property, has 
been worked out by myself, as my note-books will abundantly 
tesufy. RICHARD A, PROCTOR 
Brighton, August 28 
A Rare Phenomenon 
SUNDAY, the 13th August, and several days before, having been 
very hot and dry, a great deal of dust was suspended in the atmo- 
sphere, which caused without doubt the intense red colour of the 
setting sun, and might contribute to the phenomenon I am about 

| to describe. This phenomenon may easily be understood by 
means of a globe bisected by a meridian plane, one half of it 
representing the celestial yault. Beginning at the eastern end of 
| the equator, the space between the goth and soth degree of 
longitude may be tinged with reddish grey ; then the space be- 
| tween the 60th and 75th degree, further, that between the 

