64 
Ir is often supposed that the reality of alchemy, the transfor- 
mation of the base into the noble metals, was generally accepted 
by orientals. But, according to Herr E. Wiedemann (Amn, der 
Phys. No. 10), some of the most noted sazanés rejected the 
idea. In his Prolegomenon, Ibn Khaldiin maintains that the 
transformation of metals is impossible, the philosopher’s stone 
eannot exist, and the study of alchemy is ruinous. His own 
wiews, however, iuterest us less than his citation of Avicenna 
and his school as opponents of alchemy. While Abn Nasir al 
Farabi, an older philosopher, held that all metals belong to the 
same species, and differed oaly in accidents, so that a transforma- 
tion of these into each other was possible, Avicenna maintained 
that the metals differed in species, and that their specific differ- 
ences, ordained by Ged, were therefore not alterable by chemical 
operations. A noted alchemist, Togair, contended, against this, 
that the task of alchemy was not to impart these differences to 
metals, but only to alter the latter so that they might be enabled 
to acquire them ; the means to this being the elixir. A great 
predecessor of Avicenna, Al Kindi, also appears to have opposed 
alchemy. 
THE additions to the Zoological Society’s Gardens during the 
past week include two Vulpine Phalangers (Pha/angista vulpina), 
a Rufous Rat Kangaroo (Ayfsifrymnus rufescens) from Aus- 
tralia, presented by Mr. F. J. Horniman, F.Z.S. ; a Ring-necked 
Parrakeet (Palcornis torguata) from India, presented by the 
Countess Dowager of Lonsdale; two Long-eared Owls (Asie 
otus), British, presented by the Rev. J. A. Wix; two Grey 
Wagtails (Motacilla sulpkurea), British, presented by Mr. 
Swaysland ; a Dufresne’s Amazon (Chrysotis dufresniana) from 
South-East Brazil, a Yellow-cheeked Amazon (Chrysotts autum- 
aalis) from Honduras, an Orange-winged Amazon (Cf4rysotis 
amazonica) from South America, deposited ; a Pluto Monkey 
(Cercopithecus pluto), a Sykes’s Monkey (Cercopithecus albogu- 
Jeris) from West Africa, 2 Darwin's Rhea (Rica darwini) from 
Patagonia, a Picazuro Pigeon (Columba picaruro) from South 
America, two Spotted Zenaida Doves (Zenaida maculata) from 
La Plata, two Dominican Gulls (Zarus dominicanus) from 
Antaretic America, purchased. 
EXPERIMENTS ON COLOUR? 
IX a former paper with the above title (NATURE, vol. iii. p. 
234) I described some combinations of absorbing media 
capable of transmitting the red and green, while stopping the 
other rays of the spectram. In this way-I obtained a purely 
compound yellow, made up of red and green, and free from 
homogeneous yellow light. In devising such combinations we 
have in the first place to seek an absorbing agent capable of 
removing the yellow of the spectrum, while allowing the red 
and green to pass. For this purpose I used an alkaline infasion 
of litums, or solution of chloride of chromium, placed in a 
trough with parallel glass sides. In order to stop the blue rays 
we may avail ourselves of chromate of potassium. If a second 
trough be not objected to, it is beSt to use the bichromate, as 
exercising the most powerful absorption upon the upper end of | 
the spectrum ; but the bichromate cannot be mixed with litmus 
without destroying the desired action of the latter upon yellow. 
In this case we must content ourselves 
chromate. 
During the last year and a half I have resumed these experi- 
ments with the view, if possible, of finding solid media capable 
of the same effects, and so of dispensing with the somewhat 
troublesome troughs necessary for fluids. With this object we 
may employ films of gelatine or of collodion, spread upon glass 
and impregnated with various dyes, gelatine being chosen when 
the dye is solable in water, and collodion when the dye is soluble 
in alcohol Thus in the case of litmus a slightly warmed plate 
is coated with a hot and carefully filtered solution of gelatine, 
allowed to remain in a perfectly horizontal position until the 
gelatine is set, and then put aside to dry, by preference in a 
* Read before Section A of the British Association, September a, r8Sx, by 
Lord Rayleigh, F.R.S. 
with the neutral | 
NATURE 
(Nov. 17, 188 
current of warm air, The films thus otained are usually some- 
what rough upon the surface, s> that I have preferred to use two 
pieces cemented together, coated sides inwards, with 
balsam. In conjunction with the litmus wemay employ 2 silver- 
stained orang g and so isolate the red and rays. For 
the orange glass Mr. C. Horner has substituted a film of col- 
lodion stained with aurine. Samples possibly vary ; but 
which I have used, though extremely opaque to the blue-green 
rays, and therefore so far very suitable for the purpose, allows a 
considerable quantity of the higher blue to pass. By —a 
aurine upon a pale yellow glass, I obtained a very perfect ab- 
sorption of the blue-green and higher rays. Plates prepared as 
above described answer the purpose very well; but I have found 
that in some cases the litmus in contact with the balsam becomes 
slowly reddened, the action creeping inwards from the pr ont AD 
dye, capable of replacing litmus, and free from this d is 
**soluble aniline blue,’’ whose absorption, as I found rather 
unexpectedly, begins in the yellow and orange. Bichromate of 
potash and aniline blue may be mixed in the same solution, and — 
there is no difficulty in so adjusting the proportions as to secure 
a good compound yellow. To obtain solid films gelatine must 
be used, as in the case of litmus, for the dye is not soluble in 
collodion. With aniline there is no difficulty from the Canada 
balsam, and two plates cemented together answer > 
For systematic observations on compound colours 
probably can be better than Maxwell’s colour box in its origi 
form ; but it seemed to me that for the examination of certain 
special questions a more portable arrangement would be con- 
venient. In an instrument of this class a full d of bright- 
ness requires that the width of the eye-slit, placed where the 
spectrum is formed, should not contract the aperture of the eye, 
t.¢. should not be less than about one-fifth of an inch; and 
although the maximum of brightness is not necessary, considera- 
tions of this kind largely influence the design. If we regard the 
width of the eye-slit as given, a certain length of spectrum is 
necessary in order to attain the desired standard in respect of 
purity of colour ; so that what we have to aim at is a sufficient 
linear extension of the spectrum. A suitable compromise can 
then be made between the claims of brightness and purity. 
The necessary length of spectrum can be obtained by i i 
either the angular dispersion of the prisms or the focal 
the lens by which the image is formed. If portability no 
object, the latter is the preferable method, and the focal length 
may well be increased up to five or six feet ; in this way we may 
obtain a field of view of given purity of colour and of maximum 
brightness, at the expense only of its angular extent. If, how- 
ever, we desire an instrument which can be moved from one 
place to another without losing its adjustment, the focal len 
of the lenses must be kept down, and then a large prismatic dis- 
persion is the only alternative. 
Increased dispersion can of course be obtained by multiplica- 
tion of prisms; but for the purpose in view, high resolving 
power is not wanted, and our object may be attained with a 
comparatively small total thickness of glass, either by the use of 
higher angles than usual, or by giving the light a more nearly 
grazing emergence. The latter was the course adopted in de- 
signing the first instrument of which I have to speak. A pair 
of prisms of 60°, cut from an ordinary single 1} x 1}-inch prism 
along a plane bisecting at right anzles its refracting edge, were 
arranged in the corner of a shallow box, so as to form what 
Thollon calls a cougle, Considered as a simple, rigidly con- 
nected refractor, the pair of prisms are placed so as to give 
minimum deviation, but the incident and emergent light makes 
smaller angles with the final surface, than if each prism were 
adjusted separately for minimum deviation. The collimati 
and focussing lenses are common spectacle glasses of about 
focus. The box is 12” x 12” x Light entering at a slit on 
one of the sides of the box would be turned by the prisms 
through an angle rather greater than a right angle, and throw a 
pure spectrum upon another side of the box. This side is cut 
away, and provided with movable screens of cardboard, so that 
any part may be open or closed as desired. When the eye is 
applied to the first slit, the prisms are seen uniformly illumin- 
ated with colours whose composition depends upon the situation 
and width of the slits between the cardboard screens through 
which light is allowed to enter. In this way we may obtain a 
uniform field of view lighted with any combination of spectral 
colours. My object, however, was to obtain an instrument for 
making comparisons between the simple and Sar mg yellow, 
and for this purpose an addition was necessary. is consisted 
