108 
NATURE 
[Vov. 30, 1882 
A circular scale graduated to degrees, with its centre 
just below the centre of the coil and its plane horizontal, 
is placed with its zero point on a line drawn on the mirror 
bottom of the box at right angles to the plane of the 
coil, so that when the needle and coil are in the magnetic 
meridian the index may point to zero, The accuracy of 
the adjustment of the zero point is to be tested by finding 
whether the same current produces equal deflections on 
the two sides of zero. To test whether tie centre of this 
divided circle is accurately under the centre of the needle 
supposed at the centre of the coil, draw from the point 
immediately under the centre of the needle two radial 
lines on the mirror bottom, one on each side of the zero 
point and 45° from it, and turn the needle round without 
giving it any motion of translation. If the index lies 
along these two radial lines when its point is at the corre- 
sponding division on the circle the adjustment is correct. 
When taking readings the observer places his eye so as 
to see the index iust cover its image in the mirror bottom 
of the box, and reads off the number of degrees and frac- 
tion of a degree, indicated on the scale by the position of 
the index. Error from parallax is thus avoided. 
A mirror with attached magnets may be used, as in the 
magnetometer, instead of the needle and index. When 
this arrangement is employed the coil is in the magnetic 
meridian, when equal deflections of the spot of light on 
the scale on the two sides of zero are observed. These 
scales, as has been already remarked, should always be 
carefully glued toa wooden, piece instead of being, as they 
frequently are, fixed with drawing pins.! 
ANDREW GRAY 
(To be continued.) 
PROFESSOR HENRY DRAPER, M.D. 
pee late Professor Henry Draper, whose death we 
announced last week, was born in Virginia in 
1837, but three years later removed to New York, at the 
time when his father, Prof. J. W. Draper, was appointed 
to the Chair of Chemistry in New York University. At 
this University Dr. Draper was educated, graduating 
in Medicine in 1858, after which he travelled abroad. In 
1860 he was elected to a professorship in his own Uni- 
versity, which he retained till his death the other day. In 
1866 he was elected Professor of Physiology in the 
Medical Department of the University and managing 
officer of the institution, a position he resigned in 1873. 
Dr. Draper's scientific work began with a series of expe- 
riments in 1857 on the function of the spleen, carried out 
by the aid of microscopic photography, an art then in its 
infancy. On his return from Europe, stimulated by a 
visit paid to Lord Rosse’s 6-foot reflector, he began the 
construction of a 154-inch reflecting telescope, and with 
this, when completed, he took photographs of the moon. 
A full account of the methods of grinding and polishing 
reflecting mirrors and the system of testing them was 
printed in 1864 in the Smithsonian “ Contributions to 
Science.”’ 
Dr. Henry Draper subsequently constructed an equa- 
torial reflecting telescope of 28 inches aperture, making 
both the mounting and the silvered glass speculum him- 
self. The object for which this instrument was intended, 
and which it succeeded in accomplishing in 1872, was 
photographing the spectra of the stars. a work which has 
been carried on with such success by Dr. Huggins in this 
country. Since the invention of the gelatino-bromide 
dry process the difficulties of this research have much 
decreased ; all the more credit is therefore due to Draper 
and the other pioneers in this branch of inquiry ; he had 
taken more than a hundred spectra of various stars. 
In 1872 Dr. Draper produced a photograph of the dif- 
fraction spectrum of great excellence. It comprised the 
2 ERRATUM.—In the preceding yart of this art cle, p. zo, col. x, line 24 | 
from top, fcr 27 read >. 
region from below G, wave length 4350, to O, wave length 
3440, on one plate. 
In 1874 Draper was appointed by the United States 
Transit of Venus Commission, Superintendent of its 
Photographic Department, and his duties in this con- 
nection were so satisfactorily performed, that in the fall 
of that year the United States Government caused a 
special gold medal to be struck in his honour at the Mint 
in Philadelphia, bearing the inscription, ‘‘ Decori becus 
Addit Avito.” This was the first time that such a public 
recognition had ever been accorded to a scientific man in 
the United States by the Government. 
In 1877 Dr. Draper printed his paper on the ‘ Dis- 
covery of Oxygen in the Sun and a New Theory of the 
Solar Spectrum.” This research has given rise to 
as much interest as any in recent times ; whatever 
the future verdict may be upon it, it was the result 
of several years’ work and most costly and elaborate 
apparatus. In 1877 Dr. Draper went to the Rocky 
Mountains, and made experiments on the transparency 
and steadiness of the atmosphere at elevations up to 
11,000 feet. In the succeeding summer he took a party 
into the same region to observe the total eclipse 
of the sun, and was fortunate enough to photograph the 
diffraction spectrum of the solar corona, which on this 
occasion was shown to be continuous. 
During the last autumnand winter he took photographs 
of the nebula in Orion. These were the first he ever made, 
and required an exposure in the telescope up to 140 
minutes, even when the most sensitive of Eastman’s 
gelatine plates were used. 
Dr. Draper's work has been done mainly at his obser- 
vatory at Hastings-on-Hudson, and at his laboratory in 
New York. In the former he had three large telescopes, 
Dr. Draper’s genial nature won him many friends 
and many English men of science well know the hos- 
pitable home at Dobb’s Ferry. These and many more 
will sympathise with Mrs. Draper in the loss which not 
only she but science has sustained in the death of so 
earnest a seeker after truth. 
THE COMET 
WE have received the following communications on 
this subject :— 
The latest information indicates that the September 
comet was first seen on the 3rd of that month at 
Auckland. 
The sketch, No. 1, represents the appearance of the 
spectrum of this comet on October 15 and 16, and sub- 
sequent mornings. The spectroscope used was one of 
Browning’s direct-vision, with five prisms. It was at- 
tached to the comet-seeker, which has a 4-inch object- 
glass, the focal length of the instrument making a distinct 
general view of the spectrum easy. As the spectroscope 
was not furnished with any means of comparing spectra, 
the positions of the bands, as shown in the sketch, were 
obtained by adjusting the viewing telescope so that each 
band was, in succession, just in the edge of the field, 
clamping the telescope, and then viewing the spectrum of 
acandle. This operation was repeated several times on 
October 16, and subsequently on the 25th. The position of 
the band in the orange-yellow was referred directly to the 
sodium line in the candle-flame. The band inthe middle 
of the green was much the brightest, and on the least 
refrangible side was sharply defined ; but, in the other 
direction, gradually diminished in brightness. When the 
slit of the spectroscope was gradually closed, the light 
was gradually diminished, but no separate line made its 
appearance, as the well-defined edge of the band would 
have led one to expect. 
The other two bands were of about equal brightness ; 
both of thera fading rapidly on the more refrangible side, 
but much more slowly in the other direction. 
