_First Annual Report_.
Dr. Henry Draper, in 1872, was the first to photograph the lines of a
stellar spectrum. His investigation, pursued for many years with great
skill and ingenuity, was most unfortunately interrupted in 1882 by his
death.
The recent advances in dry-plate photography have vastly increased our
powers of dealing with this subject. Early in 1886, accordingly, Mrs.
Draper made a liberal provision for carrying on this investigation at
the Harvard College Observatory, as a memorial to her husband. The
results attained are described below, and show that an opportunity is
open for a very important and extensive investigation in this branch of
astronomical physics. Mrs. Draper has accordingly decided greatly to
extend the original plan of work, and to have it conducted on a scale
suited to its importance. The attempt will be made to include all
portions of the subject, so that the final results shall form a complete
discussion of the constitution and conditions of the stars, as revealed
by their spectra, so far as present scientific methods permit. It is
hoped that a greater advance will thus be made than if the subject was
divided among several institutions, or than if a broader range of
astronomical study was attempted.
It is expected that a station to be established in the southern
hemisphere will permit the work to be extended so that a similar method
of study may be applied to stars in all parts of the sky. The
investigations already undertaken, and described below more in detail,
include a catalogue of the spectra of all stars north of--24° of the
sixth magnitude and brighter, a more extensive catalogue of spectra of
stars brighter than the eighth magnitude, and a detailed study of the
spectra of the bright stars.
This last will include a classification of the spectra, a determination
of the wave lengths of the lines, a comparison with terrestrial spectra,
and an application of the results to the measurement of the approach and
recession of the stars. A special photographic investigation will also
be undertaken of the spectra of the banded stars, and of the ends of the
spectra of the bright stars.
The instruments employed are an eight inch Voigtlander photographic
lens, reground by Alvan Clark & Sons, and Dr. Draper's 11 inch
photographic lens, for which Mrs. Draper has provided a new mounting and
observatory. The 15 inch refractor belonging to the Harvard College
Observatory has also been employed in various experiments with a slit
spectroscope, and is again being used as described below. Mrs. Draper
has decided to send to Cambridge a 28 inch reflector and its mountings,
and a 15 inch mirror, which is one of the most perfect reflectors
constructed by Dr. Draper, and with which his photograph of the moon was
taken. The first two instruments mentioned above have been kept at work
during the first part of every clear night for several months. It is now
intended that at least three telescopes shall be used during the whole
night, until the work is interrupted by daylight.
The spectra have been produced by placing in front of the telescope a
large prism, thus returning to the method originally employed by
Fraunhofer in the first study of stellar spectra. Four 15° prisms have
been constructed, the three largest having clear apertures of nearly
eleven inches, and the fourth being somewhat smaller. The entire weight
of these prisms exceeds a hundred pounds, and they fill a brass cubical
box a foot on each side. The spectrum of a star formed by this apparatus
is extremely narrow when the telescope is driven by clockwork in the
usual way. A motion is accordingly given to the telescope slightly
differing from that of the earth by means of a secondary clock
controlling it electrically. The spectrum is thus spread into a band,
having a width proportional to the time of exposure and to the rate of
the controlling clock.
This band is generally not uniformly dense. It exhibits lines
perpendicular to the refracting edge of the prism, such as are produced
in the field of an ordinary spectroscope by particles of dust upon the
slit. In the present case, these lines may be due to variations in the
transparency of the air during the time of exposure, or to instrumental
causes, such as irregular running of the driving clock, or slight
changes in the motion of the telescope, resulting from the manner in
which its polar axis is supported.
These instrumental defects may be too small to be detected in ordinary
micrometric or photographic observations, and still sufficient to affect
the photographs just described.
A method of enlargement has been tried which gives very satisfactory
results, and removes the lines above mentioned as defects in the
negatives. A cylindrical lens is placed close to the enlarging lens,
with its axis parallel to the length of the spectrum. In the apparatus
actually employed, the length of the spectrum, and with it the
dispersion, is increased five times, while the breadth is made in all
cases about four inches. The advantage of this arrangement is that it
greatly reduces the difficulty arising from the feeble light of the
star. Until very lately, the spectra in the original negatives were made
very narrow, since otherwise the intensity of the starlight would have
been insufficient to produce the proper decomposition of the silver
particles. The enlargement being made by daylight, the vast amount of
energy then available is controlled by the original negative, the action
of which may be compared to that of a telegraphic relay. The copies
therefore represent many hundred times the original energy received from
the stars. If care is not taken, the dust and irregularities of the film
will give trouble, each foreign particle appearing as a fine spectral
line.
Our methods of enlargement have been considered, and some of them tried,
with the object of removing the irregularities of the original spectra
without introducing new defects. For instance, the sensitive plate may
be moved during the enlargement in the direction of the spectral lines;
a slit parallel to the lines may be used as the source of light, and the
original negative separated by a small interval from the plate used for
the copy; or two cylindrical lenses may be used, with their axes
perpendicular to each other. In some of these ways the lines due to dust
might either be avoided or so much reduced in length as not to resemble
the true lines of the spectrum.
The 15 inch refractor is now being used with a modification of the
apparatus employed by Dr. Draper in his first experiments--a slit
spectroscope from which the slit has been removed. A concave lens has
been substituted for the collimator and slit, and besides other
advantages, a great saving in length is secured by this change. It is
proposed to apply this method to the 28 inch reflector, thus utilizing
its great power of gathering light.
[A description of an accompanying plate here follows, which is omitted,
as the plate cannot be easily reproduced for ordinary press printing.]
The results to be derived from the large number of photographs already
obtained can only be stated after a long series of measurements and a
careful reduction and discussion of them. An inspection of the plates,
however, shows some points of interest. A photograph of _a Cygni_, taken
November, 26, 1886, shows that the H line is double, its two components
having a difference in wave length of about one ten-millionth of a
millimeter. A photograph of _o Ceti_ shows that the lines G and _h_ are
bright, as are also four of the ultra-violet lines characteristic of
spectra of the first type. The H and K lines in this spectrum are dark,
showing that they probably do not belong to that series of lines. The
star near _[chi]' Orionis_, discovered by Gore, in December, 1885, gives
a similar spectrum, which affords additional evidence that it is a
variable of the same class as _o Ceti_. Spectra of _Sirius_ show a large
number of faint lines besides the well-known broad lines.
The dispersion employed in any normal map of the spectrum may be
expressed by its scale, that is, by the ratio of the wave length as
represented to the actual wave length. It will be more convenient to
divide these ratios by one million, to avoid the large numbers otherwise
involved. If one millionth of a millimeter is taken as the unit of wave
length, the length of this unit on the map in millimeters will give the
same measure of the dispersion as that just described. When the map is
not normal, the dispersion of course varies in different parts. It
increases rapidly toward the violet end when the spectrum is formed by a
prism. Accordingly, in this case the dispersion given will be that of
the point whose wave length is 400.
This point lies near the middle of the photographic spectrum when a
prism is used, and is not far from the H line. The dispersion may
accordingly be found with sufficient accuracy by measuring the interval
between the H and K lines, and dividing the result in millimeters by
3.4, since the difference in their wave lengths equals this quantity.
The following examples serve to illustrate the dispersion expressed in
this way: Angstrom, Cornu, 10; Draper, photographer of normal solar
spectrum, 3.1 and 5.2; Rowland, 23, 33, and 46; Draper, stellar spectra,
0.16; Huggins, 0.1.
The most rapid plates are needed in this work, other considerations
being generally of less importance. Accordingly, the Allen and Rowell
extra quick plates have been used until recently. It was found, however,
that they were surpassed by the Seed plates No. 21, which were
accordingly substituted for them early in December. Recognizing the
importance of supplying this demand for the most sensitive plates
possible, the Seed Company have recently succeeded in making still more
sensitive plates, which we are now using. The limit does not seem to be
reached even yet. Plates could easily be handled if the sensitiveness
were increased tenfold. A vast increase in the results may be
anticipated with each improvement of the plates in this respect.
Apparatus for testing plates, which is believed to be much more accurate
than that ordinarily employed, is in course of preparation. It is
expected that a very precise determination will be made of the rapidity
of the plates employed. Makers of very rapid plates are invited to send
specimens for trial.
The photographic work has been done by Mr. W.P. Gerrish, who has also
rendered important assistance in other parts of the investigation. He
has shown great skill in various experiments which have been tried, and
in the use of various novel and delicate instruments. Many of the
experimental difficulties could not have been overcome but for the
untiring skill and perseverance of Mr. George B. Clark, of the firm of
Alvan Clark & Sons, by whom all the large instruments have been
constructed.
The progress of the various investigations which are to form a part of
this work is given below:
1. _Catalogue of Spectra of Bright Stars_.--This is a continuation of
the work undertaken with the aid of an appropriation from the Bache
fund, and described in the Memoirs of the American Academy, vol. xi., p.
210. The 8 inch telescope is used, each photograph covering a region of
10° square. The exposures for equatorial stars last for five minutes,
and the rate of the clock is such that the spectra have a width of about
0.1 cm. The length of the spectra is about 1.2 cm. for the brighter, and
0.6 cm. for the fainter stars. The dispersion of the scale proposed
above is 0.1.
The spectra of all stars of the sixth magnitude and brighter will
generally be found upon these plates, except in the case of red stars.
Many fainter blue stars also appear. Three or four exposures are made
upon a single plate. The entire sky north of -24° would be covered
twice, according to this plan, with 180 plates and 690 exposures. It is
found preferable in some cases to make only two exposures; and when the
plate appears to be a poor one, the work is repeated. The number of
plates is therefore increased. Last summer the plates appeared to be
giving poor results. Dust on the prisms seemed to be the explanation of
this difficulty. Many regions were reobserved on this account. The first
cycle, covering the entire sky from zero to twenty-four hours of right
ascension, has been completed.
The work will be finished during the coming year by a second cycle of
observations, which has already been begun. The first cycle contains 257
plates, all of which have been measured, and a large part of the
reduction completed. 8,313 spectra have been measured on them, nearly
all of which have been identified, and the places of a greater portion
of the stars brought forward to the year 1900, and entered in catalogue
form. In the second cycle, 64 plates have been taken, and about as many
more will be required. 51 plates have been measured and identified,
including 2,974 spectra. A study of the photographic brightness and
distribution of the light in the spectra will also be made.
The results will be published in the form of a catalogue resembling the
Photometric Catalogue given in volume xiv. of the Annals of Harvard
College Observatory. It will contain the approximate place of each star
for 1900, its designation, the character of the spectrum as derived from
each of the plates in which it was photographed, the references to these
plates, and the photographic brightness of the star.
2. _Catalogue of Spectra of Faint Stars_.--This work resembles the
preceding, but is much more extensive. The same instrument is used, but
each region has an exposure of an hour, the rate of the clock being such
that the width of the spectrum will be as before 0.1 cm. Many stars of
the ninth magnitude will thus be included, and nearly all brighter than
the eighth. In one case, over three hundred spectra are shown on a
single plate. This work has been carried on only in the intervals when
the telescope was not needed for other purposes. 99 plates have,
however, been obtained, and on these 4,442 spectra have been measured.
It is proposed to complete the equatorial zones first, gradually
extending the work northward. In all, 15,729 spectra of bright and faint
stars have been measured.
3. _Detailed Study of the Spectra of the Brighter Stars_.--This work has
been carried on with the 11 inch photographic telescope used by Dr.
Draper in his later researches. A wooden observatory was constructed
about 20 feet square. This was surmounted by a dome having a clear
diameter of 18 feet on the inside. The dome had a wooden frame, sheathed
and covered with canvas. It rested on eight cast iron wheels, and was
easily moved by hand, the power being directly applied. Work was begun
upon it in June, and the first observations were made with the telescope
in October.
Two prisms were formed by splitting a thick plate of glass diagonally.
These gave such good results that two others were made in the same way,
and the entire battery of four prisms is ordinarily used. The safety and
convenience of handling the prisms is greatly increased by placing them
in square brass boxes, each of which slides into place like a drawer.
Any combination of the prisms may thus be employed. As is usual in such
an investigation, a great variety of difficulties have been encountered,
and the most important of them have now been overcome.
4. _Faint Stellar Spectra_.--The 28 inch reflector will be used for the
study of the spectra of the faint stars, and also for the fainter
portions near the ends of the spectra of the brighter stars. The form of
spectroscope mentioned above, in which the collimator and slit are
replaced by a concave lens, will be tried. The objects to be examined
are, first, the stars known to be variable, with the expectation that
some evidence may be afforded of the cause of the variation. The stars
whose spectrum is known to be banded, to contain bright lines, or to be
peculiar in other respects, will also be examined systematically.
Experiments will also be tried with orthochromatic plates and the use of
a colored absorbing medium, in order to photograph the red portions of
the spectra of the bright stars. Quartz will also be tried to extend the
images toward the ultra-violet.
5. _Absorption Spectra_.--The ordinary form of comparison spectrum
cannot be employed on account of the absence of a slit. The most
promising method of determining the wave lengths of the stellar spectra
is to interpose some absorbent medium. Experiments are in progress with
hyponitric fumes and other substances. A tank containing one of these
materials is interposed and the spectra photographed through it. The
stellar spectra will then be traversed by lines resulting from the
absorption of the media thus interposed, and, after their wave lengths
are once determined, they serve as a precise standard to which the
stellar lines may be referred. The absorption lines of the terrestrial
atmosphere would form the best standard for this purpose if those which
are sufficiently fine can be photographed.
6. _Wave Lengths_.--The determination of the wave lengths of the lines
in the stellar spectra will form an important part of the work which has
not yet been begun. The approximate wave lengths can readily be found
from a comparison with the solar spectrum, a sufficient number of solar
lines being present in most stellar spectra. If, then, satisfactory
results are obtained in the preceding investigation, the motion of the
stars can probably be determined with a high degree of precision. The
identification of the lines with those of terrestrial substances will of
course form a part of the work, but the details will be considered
subsequently.
From the above statement it will be seen that photographic apparatus has
been furnished on a scale unequaled elsewhere. But what is more
important, Mrs. Draper has not only provided the means for keeping these
instruments actively employed, several of them during the whole of every
clear night, but also of reducing the results by a considerable force of
computers, and of publishing them in a suitable form. A field of work of
great extent and promise is open, and there seems to be an opportunity
to erect to the name of Dr. Henry Draper a memorial such as heretofore
no astronomer has received. One cannot but hope that such an example may
be imitated in other departments of astronomy, and that hereafter other
names may be commemorated, not by a needless duplication of unsupported
observatories, but by the more lasting monuments of useful work
accomplished.