Part 11

Considerations drawn from photometry were also lost sight of, because that art was still undeveloped. Kepler saw that the sun might well be of the nature of a star; in fact, that the stars were probably suns. Had he and his contemporaries known that the light of the sun was more than ten thousand million times that of a bright star, they would have seen that it must be placed at one hundred thousand times its present distance to shine as a bright star. If, then, the stars are as bright as the sun, they must be one hundred thousand times as far away, and their annual parallax would then have been too small for detection with the instruments of the time. Such considerations as this would have removed the real difficulty.

The efforts to discover stellar parallax were, of course, still continued. Bradley, about 1740, made observations on γ Draconis, which passed the meridian near his zenith, with an instrument of an accuracy before unequalled. He thus detected an annual swing of 20″ on each side of the mean. But this swing did not have the right phase to be due to the motion of the earth; the star appeared at one or the other extremity of its swing when it should have been at the middle point, and _vice versa_. What he saw was really the effect of aberration, depending on the ratio of the velocity of the earth in its orbit to the velocity of light. It proved the motion of the earth, but in a different way from what was expected. All that Bradley could prove was that the distances of the stars must be hundreds of thousands of times that of the sun.

An introductory remark on the use of the word parallax may preface a statement of the results of researches now to be considered.

In a general way, the change of apparent direction of an object arising from a change in the position of an observer is termed _parallax_. More especially, the parallax of a star is the difference of its direction as seen from the sun and from that point of the earth’s orbit from which the apparent direction will be changed by the greatest amount. It is equal to the angle subtended by the radius of the earth’s orbit, as seen from the star. The simplest conception of an arc of one second is reached by thinking of it as the angle subtended by a short line at a distance of two hundred and six thousand times its length. To say that a star has a parallax of 1″ would therefore be the same thing as saying that it was at a distance of a little more than two hundred thousand times that of the earth from the sun. A parallax of one-half a second implies a distance twice as great; one of one-third, three times as great. A parallax of 0″20 implies a distance of more than a million times that of our unit of measure.

The first conclusive result as to the extreme minuteness of the parallax of the brighter stars was reached by Struve, at Dorpat, about 1830. In the high latitude of Dorpat the right ascension of a star can be determined with great precision, not only at the moment of its transit over the meridian, but also at transit over the meridian below the pole, which occurs twelve hours later. He, therefore, selected a large group of stars which could be observed twice daily in this way at certain times of the year, and made continuous observations on them through the year. It was not possible, by this method, to certainly detect the parallax of any one star. What was aimed at was to determine the limit of the average parallax of all the stars thus observed. The conclusion reached was that this limit could not exceed one-tenth of a second and that the average distance of the group could not, therefore, be much less than two million times the distance of the sun; if, perchance, some stars were nearer than this, others were more distant.

By a singular coincidence, success in detecting stellar parallax was reached by three independent investigators almost at the same time, observing three different stars.

To Bessel is commonly assigned the credit of having first actually determined the parallax of a star with such certainty as to place the result beyond question. The star having the most rapid proper motion on the celestial sphere, so far as known to Bessel, was 61 Cygni, which is, however, only of the fifth magnitude. This rapid motion indicated that it was probably among the stars nearest to us, much nearer, in fact, than the faint stars by which it is surrounded.

After several futile attempts, he undertook a series of measurements with a heliometer, the best in his power to make, in August, 1837, and continued them until October, 1838. The object was to determine, night after night, the position of 61 Cygni, relative to certain small stars in its neighborhood. Then he and his assistant, Sluter, made a second series, which was continued until 1840. All these observations showed conclusively that the star had a parallax of about 0″.35.

While Bessel was making these observations, Struve, at Dorpat, made a similar attempt upon Alpha Lyræ. This star, in the high northern latitude of Dorpat, could be accurately observed throughout almost the entire year. It is one of the brightest stars near the Pole and has a sensible proper motion. There was, therefore, reason to believe it among the nearest of the stars. The observations of Struve extended from 1835 to August, 1838, and were, therefore, almost simultaneous with the observations made by Bessel on 61 Cygni. He concluded that the parallax of Alpha Lyræ was about one-fourth of a second. Subsequent investigations have, however, made it probable that this result was about double the true value of the parallax.

The third successful attempt was made by Henderson, of England, astronomer at the Cape of Good Hope. He found from meridian observations that the star Alpha Centauri had a parallax of about 1″. This is a double star of the first magnitude, which, being only 30° from the south celestial pole, never rises in our latitudes. Its nearness to us was indicated not only by its magnitude, but also by its considerable proper motion.

Although subsequent investigation has shown the parallax of this body to be less than that found by Henderson, it is, up to the time of writing, the nearest star whose distance has been ascertained. The extreme difficulty of detecting movements so slight as those we have described, when they take six months to go through their phases, will be obvious to the reader. He would be still more impressed with it when, looking through a powerful telescope at any star, he sees how it flickers in consequence of the continual motions going on in the air through which it is seen and how difficult it must be to fix any point of reference from which to measure the change of direction.

The latter is the capital difficulty in measuring the parallax. How shall we know that a star has changed its direction by a fraction of a second in the course of six months? There must be for this purpose some standard direction from which we can measure.

The most certain of these standard directions is that of the earth’s axis of rotation. It is true that this direction varies in the course of the year, but the amount of the variation is known with great precision, so that it can be properly allowed for in the reduction of the observations. The angle between the direction of a star and that of the earth’s axis, the latter direction being represented by the celestial pole, can be measured with our meridian instruments. It is, in fact, the north polar distance of the star, or the complement of its declination. If, therefore, the astronomer could measure the declination of a star with great precision throughout the entire year, he would be able to determine its parallax by a comparison of the measures. But it is found impossible in practice to make measures of so long an arc with the necessary precision. The uncertain and changing effect of the varying seasons and different temperatures of day and night upon the air and the instrument almost masks the parallax. After several attempts with the finest instruments, handled with the utmost skill, to determine stellar parallax from the declinations of the stars, the method has been practically abandoned.

The method now practiced is that of relative parallax. By this method the standard direction is that of a small star apparently alongside one whose parallax is to be measured, but, presumably, so much farther away that it may be regarded as having no parallax. In this assumption lies the weak point of the method. Can we be sure that the smaller stars are really without appreciable parallax? Until recent times it was generally supposed that the magnitude of the stars afforded the best index to their relative distances. If the stars were of the same intrinsic brilliancy, the amount of light received from them would, as already pointed out, have been inversely as the square of the distance. Although there was no reason to suppose that any such equality really existed, it would still remain true that, in the general average, the brighter stars must be nearer to us than the fainter ones. But when the proper motions of stars came to be investigated, it was found that the amount of this motion afforded a better index to the distance than the magnitude did.

The diversity of actual or linear motion is not so wide as that of absolute brilliancy. Stars have, therefore, in recent times, been selected for parallax very largely on account of their proper motion, without respect to their brightness. It is now considered quite safe to assume that the small stars without proper motion are so far away that their parallax is insensible.

Ever since the time of Bessel the experience of practical astronomers has tended toward the conclusion that the best instrument for delicate measurements like these is the heliometer. This is an equatorial telescope of which the object glass is divided along a diameter into two semicircles, which can slide along each other. Each half of the object glass forms a separate image of any star at which the telescope may be pointed. By sliding the two halves along each other, the images can be brought together or separated to any extent. If there are two stars in proximity, the image of one star made by one-half of the glass can be brought into coincidence with that of the other star made by the other half. The sliding of the two halves to bring about this coincidence affords a scale of measurement for the angular distance of the two stars.

The most noteworthy forward steps in improving the heliometer are due to the celebrated instrument-makers of Hamburg, the Messrs. Repsold, aided by the suggestions of Dr. David Gill, astronomer at the Cape of Good Hope. The latter, in connection with his coadjutor, Elkin, made an equally important step in the art of managing the instrument and hence in determining the parallax of stars. The best results yet attained are those of these two observers and of Peter, of Germany.

Yet more recently, Kapteyn, of Holland, has applied what has seemed to be the unpromising method of differences of right ascension observed with a meridian circle. This method has also been applied by Flint, at Madison, Wis. Through the skill of these observers, as well as that of Brünnow and Ball, in applying the equatorial telescope to the same purposes, the parallax of nearly 100 stars has been measured with some approach to precision.

A rival method to that of the heliometer has been discovered in the photographic telescope. The plan of this instrument, and its application to such purposes as this, are extremely simple. We point a telescope at a star and set the clock-work going, so that the telescope shall remain pointed as exactly as possible in the direction of the star. We place a sensitized plate in the focus and leave it long enough to form an image both of the particular star in view and of all the stars around it. The plate being developed, we have a permanent record of the relative positions of the stars which can be measured with a suitable instrument at the observer’s leisure. The advantage of the method consists in the great number of stars which may be examined for parallax, and in the rapidity with which the work can be done.

The earliest photographs which have been utilized in this way are those made by Rutherfurd in New York during the years 1860 to 1875. The plates taken by him have been measured and discussed principally by Rees and Jacoby, of Columbia University. Before their work was done, however, Pritchard, of Oxford, applied the method and published results in the case of a number of stars.

One of the pressing wants of astronomy at the present time is a parallactic survey of the heavens for the purpose of discovering all the stars whose parallax exceeds some definable limit, say 0″1. Such a survey is possible by photography, and by that only. A commencement, which may serve as an example of one way of conducting the survey, has been made by Kapteyn on photographic negatives taken by Donner at Helsingfors.

These plates cover a square in the Milky Way about two degrees on the side, extending from 35° 50′ in declination to 36° 50′, and from 20h. 1m. in R. A. to 20h. 10m. 24s. Three plates were used, on each of which the image of each star is formed twelve times. Three of the twelve impressions were made at the epoch of maximum parallactic displacement, six at the minimum six months later, and three at the following maximum. The parallaxes found on the plates can only be relative to the general mean of all the other stars, and must therefore be negative as often as positive. The following positive parallaxes, amounting to 0″1, came out with some consistency from the measures:

Star, B. D., 3972 Mag. 8.6 R.A. 20h., 2m. 0s. Dec. +35°.5 Par. +0″.11

Star, B. D., 3883 Mag. 7.1 R.A. 20h., 2m. 3s. Dec. +36°.1 Par. +0″.18

Star, B. D., 4003 Mag. 9.2 R.A. 20h., 4m. 58s. Dec. +35°.4 Par. +0″.10

Star, B. D., 3959 Mag. 7.0 R.A. 20h., 9m. 14s. Dec. +36°.3 Par. +0″.10

Against these are to be set negative parallaxes of -0″.09, -0″.08 and several a little smaller, which are certainly unreal.

The presumption in favor of the actuality of one or more of the above positive values, which is created by their excess over the negative values, is offset by the following considerations: The area of the entire sky is more than 40,000 square degrees, or 10,000 times the area covered by the Helsingfors plates. We cannot well suppose that there are 1,000 stars in the sky with a parallax of 0″.10, or more without violating all the probabilities of the case. The probabilities of the case are therefore against even one star with such a parallax being found on the plates. Yet the cases of these four stars are worthy of further examination, if any of them are found to have a sensible proper motion.

On an entirely different plan is a survey just concluded by Chase with the Yale heliometer. It includes such stars having an annual proper motion of 0″.05 or more as had not already been measured for parallax. The results, in statistical form, are these:

2 stars have parallaxes between +9″.20 and +0″.25.

6 stars have parallaxes between +0″.15 and +0″.20.

11 stars have parallaxes between +0″.10 and +0″.15.

24 stars have parallaxes between +0″.05 and +0″.10.

34 stars have parallaxes between +0″.00 and +0″.05.

8 stars have parallaxes between -0″.05 and 0″.00.

5 stars have parallaxes between -0″.10 and -0″.05.

2 stars have parallaxes between -0″.15 and -0″.10. -- 92, total number of stars.

It will be understood that the negative parallaxes found for fifteen of these stars are the result of errors of observation. Assuming that an equal number of the smaller positive values are due to the same cause, and subtracting these thirty stars from the total number, we shall have sixty-two stars left of which the parallax is real and generally amounts to 0″.05, more or less. The two values approximating to 0″.25 seem open to little doubt. We might say the same of the six next in the list. The first two belong to the stars 54 Piscium and Weisse, 17h., 322.

DISCUSSION AND CORRESPONDENCE.

_THE MEETINGS OF THE AMERICAN ASSOCIATION._

The American Association for the Advancement of Science has a membership ranging from 1,900 to 2,000. Of this number probably at no one time was there an aggregate of 300 persons present at the recent annual meeting in New York.

When the Association meets in an Eastern city the attendance is generally twice if not three times as large as when it convenes in the West. So little was made of the recent meeting, locally or officially, that an intelligent resident of the city remarked: “Why, I intended to have attended some of the meetings, but seeing no reference in the daily papers, it entirely escaped my mind.”

Of the 2,000 members, about 800 are fellows; the 1,200 and more registered as members are, presumably, persons devoting little or no time to independent research along scientific lines, but persons who while not actively so engaged are more than ordinarily interested in the discussion of scientific topics. These have in the past paid dues and attended the meetings of the Association with more or less regularity. It is a question in the minds of some of the 1,200 if their attendance at the meetings is desired. Their membership, so far as it relates to the five dollars initiation fee and three dollars dues, is without question acceptable, and to persons reading papers in the various sections their presence is preferable to empty seats, but in view of the fact that during recent years the management of the Association has eliminated, so far as possible, the popular features of the general programme, the question is reasonably asked: “Does the management desire the attendance of the 1,200, or is their financial support all that is desired?”

It was stated some years ago that the purpose of the Association was to furnish not only an occasion for scientists to present original papers, but also to interest the public by holding the meetings annually in different parts of the country; but if attendance is not secured (by preparation and publication of interesting features of a programme) no great interest will be awakened by a meeting held in any part of the country.

I should like to suggest the following ways of increasing the interest of the meetings:

The general daily sessions might be made occasions of rare interest by the introduction of prominent men of science who would make at least brief remarks. This would make it possible for those who have limited time to become familiar with the faces of those whom they would like to know, and the little ‘sample’ of scientific thought thrown out would doubtless awaken desire for more.

It will be objected that the meetings of the council immediately preceding the general session prevent holding an official meeting at that hour. The public and the 1,200 would care little whether the session were official or unofficial so it were interesting and instructive.

The officers of the several sections could easily secure distinguished representatives of their respective sciences to give brief addresses followed by discussion, and thus the morning hour would prove an attraction to citizens and others who might be unable to attend the sessions following.

Again, citizens, where the meetings are held, would be pleased to provide excursions to points of local interest and extend social courtesies, if they were given in return the mental food in digestible form, with which the Association is so amply supplied.

It remains with the management to decide whether attendance shall be restricted to the few actively engaged in scientific pursuits, or whether it shall include the 1,200 and more who would be glad to avail themselves of the benefits of a programme suited to average scholarship and intellectual capacity.

There is no better medium for discussion of the above views than through the widely read pages of THE POPULAR SCIENCE MONTHLY.

M. E. D. TROWBRIDGE. _Detroit, Mich._

[The questions brought up by our correspondent have been carefully considered by all those who are interested in the American Association for the Advancement of Science. When the Association was founded fifty years ago there was no division into sections; the papers and discussions were intelligible and interesting to all members. At that time there were but few members, the scientific life of the country was small, and it was a privilege for a city to entertain the Association. But fifty years have brought changes in many directions. Specialization in science has become essential for its further progress, and it has been necessary to divide the Association into numerous sections and to found special societies. Hospitality can now only be provided at great expense, and Eastern cities no longer regard it as a privilege to entertain the numerous societies that gather within their hotels. The newspapers do not regard a meeting of the Association as an important event and will not devote space to it.

The Association must do the best it can to adapt itself to existing conditions. The recent meeting in New York had perhaps the largest attendance of scientific men of any in the history of the Association with the exception of the anniversary meeting two years ago, but New York City, especially in the month of June, is not a desirable place for social functions. It is not reasonable for a member interested in science as an amateur to expect to purchase for three dollars a week’s entertainment. His dues secure reduced railway and hotel rates; he can meet his friends and become acquainted with scientific men; he can always find on the programme papers that are of interest; he receives the annual volume of ‘Proceedings’ and the weekly journal, ‘Science,’ the cost of which is five dollars per year. But apart from these direct returns, he is surely repaid for membership by knowing that he is one of those who are united for the advancement of science in America.--EDITOR, POPULAR SCIENCE MONTHLY.]

_THE COLOR RED._

TO THE EDITOR OF THE POPULAR SCIENCE MONTHLY: Mr. Havelock Ellis, in your August number, in ‘The Psychology of Red,’ says, ‘A great many different colors are symbolical of mourning ... but so far as I am aware, red never.’ The following may possibly be of interest in this connection:

“Our English Pliny, Bartholomew Glantville, who says after Isydorus, ‘Reed clothes ben layed upon deed men in remembrance of theyr hardynes and boldnes, whyle they were in theyr bloudde.’ On which his commentator, Batman, remarks: ‘It appereth in the time of the Saxons that the manner over their dead was a red cloath, as we now use black. The red of valiauncie, and that was over kings, lords, knights and valyaunt souldiers; white over cleargie men, in token of their profession and honest life, and over virgins and matrons.’”--(Dr. Furness’s Variorum. Merchant of Venice, p. 56.)

CHAS. E. DANA. _University of Pennsylvania._

SCIENTIFIC LITERATURE.

_MENTAL AUTOMATISM._