
— May 4, 1871| 
NATURE 13 
J 

direction. The main course was from west to east. He also 
saw it at Pernambuco, at Rio Janeiro, and in the Southern 
States of America, but nowhere so abundant as on the Amazons. 
The Urania is scarcely a butterfly ; but between the day and 
night butterflies, something between a skipper and a hawk moth, 
By Latreille they were called Hespero-Sphyngide. The larve 
and pupz are supposed not to have been adequately examined. 
The Liverpool naturalist could not identify them, and as yet they 
have not been able to find themat Panama. In Central Ameriza 
the Urania is found as far north as Guatemala. Mr. Darwin 
observed a butterfly of similar habits, the Pupilio feronia, which 
frequents orange groves. 


THE ROVAL SOCIETY'S LIST FOR 1871 
“aus following fifteen have been selected by the Council 
of the Royal Society out of the fifty candidates, and 
recommended to the Fellows for election :— Ve//éam Henry 
Besant, M.A., Mathematical Lecturer at St. John’s Col- 
lege ; Senior Wrangler and First Smith’s Prizeman in 
1850, Moderator in 1856, Examiner in the University of 
London from 1860 to 1865 ; author of Treatises on “ Hy- 
dromechanics and the Theory of Sound,” 2nd ed. 1867; 
“Elementary Hydrostatics,” 2nd ed. 1867 ; “ Geometrical 
Conic Sections,” 1869 ; “ Roulettes and Glissettes,” 1870. 
William Budd, M.D. (Edin.), physician, author of various 
medical papers, especially relating to contagious diseases. 
George W. Callender, F.R.C.S., lecturer on Anatomy at St. 
Bartholomew’s Hospital School, and Assistant Surgeon 
to Bartholomew’s Hospital ; author of Anatomical papers. 
William Carruthers, ¥.L.S., F.G.S., keeper of the Botanical 
Department, British Museum ; author of “Fossil Cyca- 
dean Stems from the Secondary Rocks of Britain ;” “On 
the Structure and Affinities of Sigillaria and Allied Ge- 
nera ;” “ The Cryptogamic Forests of the Coal Period;” “On 
the Structure of the Stems of the Arborescent Lycopodiacee 
of the Coal-measures ;” “Revision of the British Grapto- 
lites,” &c. Robert Etheridge, ¥.R.S.E.,F.G.S., Palzontolo- 
gist to H.M. Geological Survey of Great Britain; Demon- 
strator on Palzeontology, Royal School of Mines; author of 
numerous geological papers. Svrederick Guthrie, B.A., 
F.R.S.E., F.C.S., Professor of Physics in the Royal School 
of Mines; author of various papers on Chemistry and 
Physics. Captain Fohn Herschel, R.E., of the Great 
Trigonometrical Survey of India. Cajftaim Alexander 
Moncrieff, Militia Artillery, C.E., inventor of the Mon- 
crieff gun-carriage, and author of the Moncrieff system 
of defence. Richard Quain, M.D. (Lond.), Fellow and 
late Censor of the Royal College of Physicians ; author 
of a paper “ On Fatty Degeneration of the Heart,” which 
has exerted a marked influence on certain branches of 
Pathological Science ; and of numerous communications 
published in the Transactions of the Pathological Society, 
of which Society he was President (1869-70). Card Schor- 
Zemmer, Senior Assistant in Owens College Laboratory, 
Manchester ; author of a series of papers on the Consti- 
tution of the Paraffins, chiefly published in the Proceedings 
of the Society since 1862. Edward Thomas, Treas. R.A.S., 
author of numerous papers on Indian Coins and Gems. 
Edward Burnet Tylor, author of “ Researches into the 
Early History of Mankind ;” “ Primitive Culture ;” and 
various memoirs on Savages and their Customs. Cyom- 
well Fleetwood Varley, Civil and Telegraphic Engineer, 
M.I.C.E.; Consulting Electrician to the Electric and 
International Telegraph Company, the Atlantic Telegraph 
Company, la Société du Cable Transatlantique Francais; 
author of many inventions in connection with the Electric 
Telegraph. Viscount Walden, President of the Zoologi- 
cal Society of London ; author of various papers on Or- 
nithology. ohn Wood, ¥.R.C.S., Examiner in Anatomy 
at the University of London ; author of a number of 
anatomical papers published in the Phil. Trans, 



ON COLOUR VISION * 
LL vision is colour vision, for it is only by observing differ- 
ences of colour that we distinguish the forms of objects. Iin- 
clude differences of brightness or shade among differences of colour. 
It was in the Royal Institution, about the beginning of this 
century, that Thomas Young made the first distinct announce- 
ment of that doctrine of the vision of colours which I propose to 
illustrate. We may state it thus :—We are capable of feeling 
three different colour-sensations. Light of different kinds 
excites these sensations in different proportions, and it is by the 
different combinations of these three primary sensations that all 
the varieties of visible colour are produced. In this statement 
there is one word on which we must fix our attention. That 
word is, Sensation. It seems almost a truism to say that colour 
is a sensation; and yet Young, by honestly recognising this 
elementary truth, established the first consistent theory of colour. 
So far as I know, Thomas Young was the first who, starting 
from the well-known fact that there are three primary colours, 
sought for the explanation of this fact, not in the nature of light, 
but in the constitution of man. Even of those who have written 
on colour since the time of Young, some have supposed that 
they ought to study the properties of pigments, and others that 
they ought to analyse the rays of light. They have sought for a 
knowledge of colour by examining something in external nature 
—something out of ourselves. 
Now, if the sensation which we call colour has any laws, it 
must be something in our own nature which determines the form 
of these laws ; and I need not tell you that the only evidence we 
can obtain respecting ourselves is derived from consciousness. 
The science of colour must therefore be regarded as essentially 
a mental science. It differs from the greater part of what is 
called mental science in the large use which it makes of the 
physical sciences, and in particular of optics and anatomy. But 
It gives evidence that it is a mental science by the numerous 
illustrations which it furnishes of various operations of the mind. 
In this place we always feel on firmer ground when we are 
dealing with physical science. I shall therefore begin by 
showing how we apply the discoveries of Newton to the mani- 
pulation of light, so as to give you an opportunity of feeling for 
yourselves the different sensations of colour. 
Before the time of Newton, white light was supposed to be of 
all known things the purest. When light appears coloured, it 
was supposed to have become contaminated by coming into con- 
tact with gross bodies. We may still think white light the 
emblem of purity, though Newton has taught us that its purity 
does not consist in simplicity. 
We now form the prismatic spectrum on the screen. These 
are the simple colours of which white light is always made up. 
We can distinguish a great many hues in passing from the one 
end to the other ; but it is when we employ powerful spectro- 
scopes, or avail ourselves of the labours of those who have 
mapped out the spectrum, that we become aware of the immense 
multitude of different kinds of light, every one of which has been 
the object of special study. Every increase of the power of our 
instruments increases in the same proportion the number of lines 
visible in the spectrum. 
All light, as Newton proved, is composed of these rays taken 
in different proportions. Objects which we call coloured when 
illuminated by white light, make a selection of these rays, and 
our eyes receive from them only a part of the light which falls on 
them. But if they receive only the pure rays of a single colour 
of the spectrum, they can appear only of that colour. If I place 
a disc containing alternate quadrants of red and green paper 
in the red rays, it appears all red, but the red quadrants brightest. 
If I place it in the green rays both papers appear green, but the 
red paper is now the darkest. This, then, is the optical expla- 
nation of the colours of bodies when illuminated with white light 
They separate the white light into its component parts, absorbing 
some and scattering others, 
Here are two transparent solutions. One appears yellow, it 
contains bighromate of potash ; the other appears blue, it con- 
tains sulphate of copper. If I transmit the light of the electric 
lamp through the two solutions at once, the spot on the screen 
appears green. By means of the spectrum we shall be able to 
explain this. The yellow solution cuts off the blue end of the 
spectrum, leaving only the red, orange, yellow, and green. The 
blue solution cuts off the red end, leaving only the green, blue, 
and violet. The only light which can get through both is the 
* Lecture delivered before the Royal Institution, March.24th. 
