OCTOBEE 4, 1912] 



SCIENCE 



419 



like Argelander, the Herschels and the 

 Bonds iindertook other lines of work, but 

 the last half of the nineteenth century saw 

 the birth of a new science, astrophysics, 

 which relates to other properties of the 

 stars, such as their brightness, size, color 

 and spectrum. Astronomy may, therefore, 

 be divided into two classes, the astronomy 

 of position and astrophysics, which Lang- 

 ley called the old and the new astronomy. 

 Two years ago, forty of the leading astron- 

 omers of Europe visited this country to 

 attend the meeting of the Solar Union, at 

 the Mount Wilson Observatory, in Cali- 

 fornia. On the way, many of them at- 

 tended the meeting in Cambridge of the 

 Astronomical and Astrophysical Society of 

 America, now in session here. One of the 

 most eminent of these astronomers, while 

 complimenting us highly on American work 

 in astrophysics, pointed out that of the 

 forty-eight papers read at the meeting, but 

 one related to astronomy of position. You 

 are fortunate in having as director of the 

 Allegheny Observatory an astronomer who 

 has distinguished himself in both depart- 

 ments of the science. A very interesting 

 problem in astronomy of position is a curi- 

 ous motion of the earth by which the lati- 

 tude of any given point is continually 

 changing by a small amount. If we found 

 the average position of the North Pole and 

 built a circular fence around it seventy 

 feet in diameter, the pole would wander 

 about the enclosed space, describing an 

 irregular spiral, but never going outside 

 of it. Last year the pole appears to have 

 been nearer the fence than at any time 

 during the last quarter of a century, or 

 since the discovery of this motion. A 

 number of stations are now maintained by 

 the International Geodetic Association, at 

 which continuous measures are made of 

 the position of the pole. The accuracy of 

 these measures is such that its position is 



known within one or two feet. For four 

 years Professor Schlesinger took part in 

 these measures at the station in Ukiah, 

 California. He thus familiarized himself 

 with some of the most accurate methods of 

 measurement of position known. 



One of the most important problems 

 before astronomers at the present time is 

 to determine the distances of the stars. 

 The only direct method of finding the dis- 

 tance of an inaccessible object is to meas- 

 ure the change in its apparent position as 

 seen from different points. Fortunately, 

 we can apply this method to the stars, since 

 the earth is more than ninety million miles 

 from the sun, and by its revolution around 

 the latter its position is changed by nearly 

 two hundred million miles. It is quite 

 impossible for the mind of man to conceive 

 of such a distance, but vast as it is, it is 

 abnost inappreciable compared with the 

 distance of the stars. Few of them are 

 less than a million times as distant as the 

 sun, and the greater portion of them are 

 probably thousands of times as distant as 

 the nearest. The apparent change in posi- 

 tion of the nearest star, as the earth moves 

 two hundred million miles, would equal the 

 height of a man at a distance of two hun- 

 dred miles. In other words, the problem is 

 like measuring the height of a man two hun- 

 dred miles away. Various methods have 

 been tried, but the most accurate of aU 

 appears to be that employed by Professor 

 Schlesinger. He finds the height of the 

 man with an uncertainty of only one inch ! 

 The method he used, when he was at the 

 Yerkes Observatory, consisted in taking 

 photographs with the forty-inch refractor 

 of that institution, the largest telescope of 

 its kind in the world. By using plates 

 sensitive to the yellow rays, he obtained 

 very minute images of the stars, which 

 could be measured with the greatest accu- 

 racy. The results for a large number of 



