on 
Oo 
right ascension. To save the strain on the 
observer this is moved at approximately the right 
rate by an eleciric motor with suitable mechanism 
for giving the rate corresponding to the declina- 
tion of the star. The great success of this in- 
strument is attested by the results already pub- 
lished by Mr. Hough. 
A full description is also given of the Victoria 
telescope, presented to the observatory by Mr. 
Frank McClean. It is furnished with a spectro- 
scope for the determination of velocity in the line 
of sight, and with two large objective prisms of 
24 in. aperture and refracting angles of 8° and 
12° respectively: These can be used singly or 
together. Excellent results, including a deter- 
mination of the solar parallax by Dr. Halm, have 
been already obtained with the line of sight 
spectroscope. The Cape Observatory is there- 
fore, thanks to the generosity of Mr. McClean, 
admirably equipped for astrophysical work. 
While the reversible transit-circle and the Vic- 
toria telescope are probably the instruments which 
will be most valuable to Sir David Gill’s suc- 
cessors, it is with the 7-in. heliometer that most 
of his own personal observing work has been 
done. This instrument was obtained in 1887 to 
supersede the 4-in. used at Ascension. With it 
(partly in cooperation with Dr. Elkin) he made 
the well-known determinations of the parallax 
of the sun and of the brightest stars of the 
southern hemisphere, remarkable alike for the 
smallness of their accidental error and the care 
with which causes of systematic error were 
eliminated. In addition, the 7-in. heliometer has 
been used in other important investigations, par- 
ticularly in the determination of the mass of 
Jupiter and the orbits of its satellites, researches 
in which two young astronomers, Mr. Bryan 
Cookson and Mr. de Sitter cooperated. 
Sir David Gill includes an account of the Cape 
Photographic Durchmusterung. Knowing that 
this survey of the southern sky was proposed, 
Prof. Kapteyn volunteered to undertake the 
arduous work of measuring the photographs and 
discussing the results. From this cooperation, a 
catalogue containing the magnitudes and approxi- 
mate positions of 450,000 stars resulted, giving a 
complete survey of the southern skies; it is noted 
that the preparation of this work first directed 
Kapteyn to the study of the problems of cosmical 
astronomy. 
A very interesting account of the Geodetic Sur- 
vey of South Africa is supplemented by an 
appendix by Dr. Wilhelm Bahn (translated from 
the Beitrige zu Geophysik) on the South African 
are of meridian. Arcs of meridian were measured 
by Lacaille and Maclear, and between 1859 and 
1862 the triangulation of the southern coast of 
Cape Colony was taken in hand. Soon after 
Gill’s appointment, he pointed out to Sir Bartle 
Frere the advantage to be gained by a com- 
prehensive. survey, and recommended a gridiron 
system of chains of principal triangulation ex- 
tending over the Cape Colony, the Orange Free 
State, Natal, and the Transvaal. This work has 
NO. 2307, VOL. 92] 
8 NATURE 
[ JANUARY 15, I914 
been carried out under the direction of Sir William 
Morris in the field, with Sir David Gill as scien- 
tific adviser, who kept constantly in view the 
service to geodesy which would be derived from 
the measurement of a large are on the 30th 
meridian of east longitude. This was afterwards 
continued through Rhodesia, and the extreme arc 
measured extends over nearly 22° from 31° 36! S. 
lat. to 9° 41’ S. lat. There are six base-line Setar 
minations along the arc, and sixty determinations 
of astronomical latitude. The measures of the 
South African arc of meridian indicate a some- 
what larger terrestrial spheroid than that of 
Clarke, and are in accordance with the results 
obtained by Hayford in the United States. The 
extension of this arc to join the Egyptian arc, and 
the connection round the eastern end of the 
Mediterranean to join the Russian are measured 
by Struve is of great scientific importance, and, 
as Sir David Gill points out, offers no very 
formidable difficulty if international cooperation 
is secured. 
In this article only portions of the volume have 
been touched upon. Sir David Gill is to be con- 
gratulated on the production of a work the his- 
torical and narrative portions of which are of 
interest to all men of science, while the technical 
portions are of the greatest value to astronomers. 
He may be assured “that others will find guidance 
and inspiration in this history of the Cape Ob- 
servatory, as he himself did in Ren s account 
of Pulkova. . W. Dyson. 
METHODS OF IDENTIFYING 
PICTURES. ° 
SCIENTIFIC 
Gig to the present time the identification oF 
works of art has been entrusted entirely 
to the art expert, who brings to bear upon the 
problem his wide experience and artistic training ; 
and, in addition, it is probable that among those 
engaged in buying and selling pictures, many 
devices kept as trade secrets are useful in 
identifying pictures. While not for a moment 
denying that the final word should lie with the 
trained art expert, it is of interest to see how 
far scientific methods can be brought to bear 
upon this problem. The first step in this direc- 
tion is a careful study of the history of pigments. 
By the examination of ancient documents, such 
as the illuminated manuscripts of the monks, 
Venetian Ducali, and the Coram Rege Rolls, it 
has been possible to plan out the history of 
pigments probably with sufficient accuracy for 
practical purposes, and to fix the dates approxi- 
mately of certain pigments which appear and 
others that disappear from the artist’s palette. 
This method, where applicable, may be regarded 
as infallible, as the presence of a pigment of a 
date more modern than the date at which the pic- 
ture is supposed to be painted proves either 
forgery or repainting , and a careful microscopic 
examination make it quite possible to tell whether 
the picture has been repainted or not. 
This, however, does not settle the authorship 
