142 

greatly add to the interest excited; it would assist people to 
understand what they see, and tend to the destruction of that 
languid curiosity so paintully evident in the faces of sightseers. 
The contents of one case would serve for one or two lectures, and 
those who listened would necessarily carry away a few new 
ideas. 
We have been viewing the question solely from its popular side, 
being convinced that it is of great importance ; other plans may 
occur to the reader, and may be well worthy of ventilation. 
Henry ULLYETT 
Colour 
SINCE the publication in NATURE of my paper on ss Colour,” 
Thave received several inquiries for references on the subject. 
These I should have given at the time, only that I wrote away 
from books ; perhaps on the principle of “better late than never,” 
the publication in NATURE of the following selection may save 
trouble to some interested in the matter. 
Helmholtz: ‘* Ueber die Theorie der zusammengesetzten Far- 
ben ;” ‘*Poggendorff’s Annalen,” Ixxxvil. p. 45; “ Philos. 
Magazine,” iv. p. 519. 
; Ataxwell ae min epacats on Colour perceived by the Eye, 
with Remarks on Colour Blindness ;” ‘‘ Edinburgh Tyansac- 
tions,” xxl. p. 275. 
Maxwell: ‘*On the Theory of Compound Colours, and the 
Relations of the Colours of the Spectrum ;” ‘‘ Phil. Trans.,” 
18609. 
Maxwell: ‘“‘ Account of Experiments on the Perception of 
Colour ;” ‘‘ Phil. Mag.” (4), xiv. p. 40. 
Miiller ; ‘* Zur Theorie der Farben ;” ‘ Pogg. Ann” vol. 139, 
p. 41l. “A , 
These are the principal original memoirs. Of books on colour 
there are very few that can be trusted. Benson’s “ Principles of 
the Science of Colour” is recommended by Prof. Maxwell. 
There is also a tolerably complete exposition of the subject in 
Helmholtz’s ‘‘ Physiologische Optik,” of which exceilent work a 
French translation has, I believe, been published. 
Have any of the readers of NATure tried a double image 
vism for exhibiting the mixtures of two colours? By the aid of 
a Nicol the proportions of the components may be varied at 
pleasure, and the combination is, in my experience, more effec- 
tive than the plate of glass referred to in the books. However, 
on account of the texture of the coloured papers or wafers, the 
mixture is not so perfect as that obtained by rotation. 
J. W. STRutTr 
A Hint to 
A SMALL optical expedient which has been of service to me 
may be new to some of your readers, and useful, on occasion, to 
those among them whose sight is as long as my own. The focal 
length of the convex lens I require for my right eye in reading is 
twelve inches, and I find that by holding a lens of 30-inch focus 
about a foot from this eye Iam enabled to see distant objects not 
only with singular distinctness, but also perceptibly magnified. 
I can read moderate-sized print at the distance of twelve feet, 
and make out the details of a church tower half a mile off nearly 
as well as with a small opera glass magnifying two and a half 
times. The greater the distance of the lens from the eye the 
greater is the magnifying power ; but beyond a certain point 
(depending on the focus of the lens and the distance of the object) 
the gain is more than neutralised by the loss of distinctness 
with eyes that deviate but slightly from the normal standard, the 
lens employed must be so weak that the gain is inappreciable. 
I presume that a lens thus held at a distance from the eye, like 
the German ‘‘Stdpfel Linse” described by Sir John Herschel, 
“realises the notion of Descartes as to the mode of action of a 
telescope, which he regarded as an enlargement or prolongation 
of the eye. For the natural cornea we substitute an artificial 
one, which is more remote from the retina, and so forms there a 
larger image.” W. T. RApDFoRD 
the Longsighted 
Lignite and Selenite 
Witt you kindly allow me to inquire whether any of your 
readers can inform me if there exists any connection between 
lignite and selenite when found together, and, if so, in what way 
the lignite assists in the formation of the crystals of calcium 
sulphate. 
NATURE ’ 



| Fune 22, 1871 

T have recently found selenite in three or four different places, 
and in each case associated with lignite, viz., in the Bracklesham 
Beds near Stubbington, in the Woolwich Beds at Dulwich, and 
in ochrey clay near Lewisham Chalk Pits. 
June 19 AN AMATEUR 
Arctic Auroras 
IN answer to your inquiry, I send you the following infor- 
mation on a Northern Light observed at Kooltook, S.W. end of 
the Baikal Lake, by Dr. Dyhoffsky. It is taken from a source 
doubtless not at your disposition (Bulletin of the Siberian section 
of the Geographical Society, 1871, No. 2) :— 
“€On October 24 (1870) evening a northern tight was observed at 
Kooltook. Itbegan at 9 P.M. witha red light, which appeared more 
and more distinc'ly from behind the mountains that border the 
landscape on the north. It wasa little towards the east from the 
magneticmeridian. Thislightnow incressedin the form of a column, 
now diminished, and at times seemed to vanish entirely. After 
nearly an hour of such waverings, the light gradually began to 
increase and get broader ; at midnight it reached its utmost in- 
tensity and development. 
‘<Tts least limits on the horizon were included between N. 59° E. 
and N. 45° W. Six columns were distinctly visible at midnight, 
reaching half the distance between the horizon and the zenith, 
the middle column was the brightest and highest, but at the same 
time the narrowest, and bordered with reddish-yellow. The 
other columns were less brilliant but far broader. When the 
middle column decreased, the western one began to increase, 
though it never reached the intensity of the middle column. The 
other columns also increased and diminished by turns; then the 
phenomenon gradually fainted away, and at three o'clock there 
remained but a ruddy light, which now, as at the beginning, was 
brighter towards the east of the meridian.” 
The same aurora was observed at different localities of Europe, 
P. KRoPporKINE 
Catherine Channel, Petersburg, May 33, 1871 
E baa 
Day Auroras in the Arctic Regions 
I CAN now answer Dr. Burder’s question regarding the appear- 
ance of the Aurora Borealis inthe Arctic Regions. The other 
evening (last Thursday) I had a conversation with a distinguished 
magnetician and Arctic explorer, and he informed me that he has 
often seen the Aurora in dvoad daylight in those regions, the colour 
invariably being crimson. This, [ hope, will once for all settle 
the apparently vexed question (face Dr. Burder) of “alleged” 
daylight Auroras. Not to repeat the entire ‘‘crusher” of Dr, 
Burder’s, I think many will now discard as ‘‘ unworthy of serious 
criticism” his cirrus-cloudy arguments. He must pardon me for 
being so unceremonious, and remember his own interesting way 
of confuting—or, better, his attempt. JOHN JEREMIAH 




SCIENCE IN PLAIN ENGLISH 
il, / 
(s tracing the development of public opinion, no period 
is more instructive than the last three hundred years ; 
and at present the review is particularly important, for we 
seem to be in a position analogous to the state of Europe 
just before the Revival of Classical Learning. We are 
evidently on the eve of great changes in principle, and 
one vital question is to consider the vaiue of classical cul- 
ture as compared with the study of science. 
The distinctive work of the thirty years (1820-1850) was 
to “diffuse useful knowledge” among the middle classes. 
Beside the establishment of mechanics’ institutions 
throughout the country, the London University was 
founded in 1828; and the British Association for the 
Advancement of Science held its first annual meeting at 
York on September 27, 1831, under the presidency of 
Earl Fitzwilliam. 
Another agency has been brought into action, more 
especially directed to the practical arts, and bringing into 
friendly competition the various nations of Europe. The 
International Exhibition of 1851 had a remarkable in- 
