548 

be visible for some considerable time, but the eye may 
be fatigued for twenty minutes with yellow light with- 
out interfering with the visibility of the red light. 
(3) The eye may be fatigued with red or green without 
altering the hue of spectral yellow. This may be 
shown by wearing red or green glass spectacles which 
are transparent to yellow. (4) When a sodium flame 
is viewed after fatigue with spectral red light it is very 
little affected in the region of the after-image, though 
the green-blue after-image is very strongly marked 
on either side of the sodium flame, when the after- 
image is larger than the flame. The fundamental 
phenomena of colour sensation are still very obscure. 
Perhaps the most difficult fact to explain away is the 
obvious simplicity of white, if it be indeed true that it 
is compounded of many tints. 
BuLLeTIN No. 1 of the Chemical Section of the 
Wellcome Tropical Research ‘Laboratories, Khartoum, 
contains a paper on the estimation of methyl alcohol 
in the presence of ethyl alcohol, by Mr. W. A. R. 
Wilks; in this paper the standard process of Thorpe 
and Holmes is slightly modified so as to increase the 
degree of accuracy. Bulletin No. 2 is a discussion of 
the applicability of papyrus to paper manufacture by 
Dr. W. Beam; it is concluded that after allowing 
for the collection and transport of the papyrus to 
Europe, there is a margin for profit for this substance 
as a raw material in the paper industry. Bulletin 
No. 3, by Dr. Beam, deals with tests for hashish, 
more particularly the test which depends upon the fact 
that the resinous matter, ‘“‘cannabinol,’’ of hashish 
produces a rich purple colour on treatment with a 
small amount of caustic alkali. The principal point 
to which attention is directed is that the extract of 
Cannabis indica does not usually respond to this test; 
the influence of soil, climate, method of cultivation, 
and curing seems to have much greater effect on the 
chemical composition of this plant than has hitherto 
been suspected. 
Pror. F. C. Lea and Mr. W. Norman Thomas have 
an article in Engineering for July 2 on the change in 
density of mild steel strained by compression beyond 
the yield point. Experiments were undertaken with a 
view of ascertaining how far changes of density occur 
in: overstrained mild steel, and whether any change in 
density occurs with time after the straining-load is 
released. Preliminary experiments definitely indicated 
that a change of density was brought about by over- 
straining, and that the time factor was important. 
Until this was recognised it was difficult to get con- 
sistent results. For results to be comparable two con- 
ditions must be observed. First, if the specimens are 
loaded considerably beyond the yield point, the final 
loads must be kept on the specimens for the same 
time; in other words, the amount of strain, and thus 
the change in density, are dependent upon the time of 
loading. Secondly, the densities should be determined 
as soon after the loading as possible. Results show 
that an increase in density occurred during a rest 
period of from thirty-five to thirty-eight days after the 
load had been removed. Experiments are suspended 
meanwhile as Birmingham University is in use as a 
hospital. 
NO. 2385, VOL. 95] 
NATURE 
[JULY 15, 1915 

WE have received from Messrs. Charles Baker a 
catalogue of microscopes and accessories, all of which 
are manufactured at their factory in ‘London, The 
list includes several new models, amongst which are 
three instruments similar to the Continental designs, 
the prices of which compare very favourably with those 
quoted abroad. Another instrument of interest is 
“the workshop metallurgical microscope.” The cata- 
logue is well produced and copiously illustrated. 

OUR ASTRONOMICAL COLUMN. 
THE STRUCTURE OF THE UNIVERSE.—If the distance 
of each star were known in addition to its position 
in the sky, our knowledge of the present structure 
of the universe would be complete. In order to deter- 
mine the change in the structure, it would be only 
necessary to know the motions of the stars. We know 
the positions of a great number of the stars and their 
motions across the line of sight; we know also the 
velocities in the line of sight of a few, and the dis- 
tances of a still smaller number. The data for the 
solution of the problem are therefore very meagre. 
Nevertheless, there are indirect methods of attacking 
the problem which may tend to lead one to an approxi- 
mate solution, and it is these methods which form 
the subject of the very instructive article which appears 
in the July number of Science Progress by Mr. H. 
Spencer Jones, of the Royal Observatory, Greenwich. 
The idea, as he states, that the centre of our system is 
occupied by an immense sun, many thousands of 
times larger and more glorious than our own sun, 
and that round about it are millions of lesser suns of 
various sizes, together forming the nucleus of an 
immense spiral nebula, of which the spiral arms coil- 
ing around the nucleus appear to us as the Milky Way, 
and that this to us immense system is but one, and 
perhaps a comparatively small, island universe 
amongst thousands or millions of other island universes 
in space, is an idea which by its magnificence appeals 
to the mind of man. What forms the substance of 
the article is.the basis of truth upon which this con- 
ception is founded, and the straightforward and clear 
way in which the author has marshalled his evidence 
makes the article of particular interest. 

Tue NEBULOUS REGION NEAR OmIcRON PeERSE!.—It is 
due to photography and the portrait lens that our 
knowledge of the large diffused nebula and the dark 
regions of the sky has been gained, and it was only 
recently that Prof. E. E. Barnard’s work in this field 
was referred to in this journal. Attention is now 
directed by him to the great nebulous region near 
Omicron Persei (Astrophysical Journal, May), a photo- 
graph of which he describes and illustrates. The 
photograph was specially taken to examine more 
closely this region for dark or partly luminous matter 
which produces the apparent vacancies. The attempt 
was successful after giving an exposure of 6 hours 
and 41 minutes with the Bruce 1o- and 6-in. telescopes. 
Prof. Barnard describes the photograph in some detail, 
and points out the association of this region with 
that of the Pleiades, of which it forms part. 
Work AT THE LOWELL OpseRVATORY.—Among the 
many interesting contributions to the American 
Museum Journal (vol. xv., No. 5), two deal with the 
fine photographic work which is being carried on at 
the Lowell Observatory, this observatory being situated 
in Arizona at an elevation of 7250 ft., the finest site 
of any existing similar institution. The articles in 
question are on the subjects, ‘Oxygen and Water on 
| Mars” and ‘‘The Photograph in Astronomy,” and are 
