THE VARIABLE VELOCITIES OF 8 CAPRICORNI AND

v SAGITTARII IN THE LINE OF SIGHT.

By W. W. CAMPBELL.

THE Velocities of these stars were found to be variable, from the second plate of each. Later photographs confirm the variations. B CAPRICORNI (a = 20h 15m, 8 — — 15° 5').

This is one of the "Bright HB" stars announced several

years ago by Harvard College Observatory, observed a bright Ha. The Hy line is dark.

and in which I The lines in the

spectrum are difficult to identify, and are not, as a rule, well defined, so that the velocities assigned above may be in error a few kilometers.

LICK OBSERVATORY,

Sept. 28, 1899.

*Measures of the same plate by Mr. Wright.

**Measures by Mr. Wright.

SUGGESTIONS FOR THE DETERMINATION OF STELLAR PARALLAX BY MEANS OF PHOTOGRAPHY I

By FRANK SCHLESINGER.

THERE are not more than twenty-five or thirty stars whose parallaxes, as now known, can be relied upon within 0.05". It is no less than sixty years since the announcements by Bessel and Henderson of measurable parallaxes for 61 Cygni and for a Centauri; yet this branch of stellar astronomy has fallen far behind others of more recent origin. We know comparatively much of the proper motions of the stars and of their relative brilliancies; the work which has been done or is now in progress at Potsdam, Poulkowa, the Lick and the Yerkes Observatories will soon make us well informed as to the motions in the line of sight of most of the bright stars.

The neglect of the important subject of stellar parallax is all the more surprising in view of the ready means we now have for its pursuit, photography and the modern heliometer. Although other methods may be useful in isolated cases, there can be little doubt that these two" methods are the best for an extensive attack upon stellar parallaxes. It would be out of place to discuss here the relative advantages of these two methods, but it may be said that while photography is not inferior in accuracy to the heliometer, it has much to recommend it for the work in view, especially on the score of economy of time. On the other hand, experience has shown that photographs are liable to peculiar errors, the means of obviating which I purpose to consider very briefly.

The star to be examined for parallax will usually exceed each of the comparision stars in brightness by 6 or 7 magnitudes; that is to say, it will be about 400 times as intense. Consequently on a plate which has been exposed long enough to bring Read at the Third Conference of Astronomers and Astrophysicists, September 8, 1899.

out the comparison stars well, the central star will be unduly broadened; not only is this fatal to exact bisection, but systematic errors are apt to creep in. Hence our first task is to cut out, in some way, most of the light of the central star; this may be done, in effect, by rendering the film less sensitive in the small area upon which the image of the star will fall. A central strip of the film may be washed with a colored fluid and then allowed to dry thoroughly before exposure. Experiment will be necessary to determine the best dye for the purpose, and how strong a solution will be necessary to reduce a star of given magnitude to say one of the ninth.

We are brought at once to a second source of error, distortion of the film after exposure. Objection will be made, and with reason, that the process just proposed might lead to considerable distortion. Consequently extra precautions will be necessary to guard against their effects. Dr. Wilsing, of Potsdam, has lately used the ingenious device of taking two pictures of the same region close together on a plate, but six months apart in time. Thus if there is any distortion of the film it will shift both pictures alike, and its effects will be eliminated from the parallax. We may modify this process slightly by taking two pictures on the same date very close together; then six months. later two more may be taken exterior to the first two, and separated from the latter in such a way that the three spaces between the four images of the same star shall be equal. By this means any linear distortion, no matter how violent, will have no effect so far as parallax is concerned.

A third source of error is optical distortion, caused by peculiarities of the object-glass. Such errors are small and only in the most refined work need they be guarded against. With equatorials of the usual mounting, in following an object across the meridian, it is necessary to turn the telescope 180° upon both the polar and hour axes. Hence if there is any optical distortion, its full effect will be involved with parallax, because photographs made at an eastern elongation of a star will nearly always be taken with the objective reversed 180°

(with respect to a configuration in the sky) from the position for a western elongation. A simple means for obviating any ill effects from this cause, is to mount the object-glass in such a way that it may be revolved in its own plane, around the axis of the telescope tube. On the latter two stops may be placed so that the corresponding positions of the object-glass shall differ by 180°. Throughout the whole series of observations the observer must be careful to reverse the object-glass whenever he reverses the telescope. In this way he will be enabled to present the objective in the same relative position to a configuration, no matter whether in the east or in the west. Optical distortion will then shift all the images of the same star alike, and will not affect the parallax.

The last source of error which will be considered here has its origin in the construction of the measuring machine. It is hardly necessary to point out that all the measures should be differential; thus, the distance between a certain pair of stars on any plate should always be measured with the same parts of the scale and of the micrometer-screw. A simple arrangement of the plan of measuring is possible, by which the parallax is rendered practically independent of most of the instrumental errors.

Other precautions are necessary in parallax work, but as they are not peculiar to photographs they need not be discussed here.

To sum up, the whole process is as follows: put the unexposed plate into a properly constructed template and stain a narrow central strip to a degree previously determined by experiment for plates of the same manufacture. Permit the plate to dry for several weeks at least. Then insert it in the plateholder of the telescope, in such a way that the stained strip shall be parallel with an hour-circle. Make two pictures of the same region, shifting the plate a millimeter in declination between the two exposures. Store the plate in a dark room, undeveloped. Six months later, with the object-glass reversed 180° if the telescope mounting demands it, take two more pictures on the plate. These should be 3 millimeters apart, and each millimeter from one of the former pictures. Begin a new

plate by making two exposures 1 millimeter apart, and put this by for six months; and so on till a sufficiently long chain of plates is secured to give good values both for the parallax and for relative proper motions, with respect to the comparison stars.

I may remark in conclusion that if we confine ourselves to the measurement of distances only, much simplification is possible in the measuring machine, and some in the reductions. According to a conservative estimate, a single observer, working fifteen or eighteen hours per week at the telescope, and employing the rest of his time in measuring and reducing, could give us in three or four years the parallaxes of 200 stars with an accuracy hitherto attained for only a score.

UKIAH, CAL.

August 29, 1899.