Aprils, 1886] 



NA TURE 



535 



fhem a large collection of apparatus employed in various 

 parts of the course was laid out. 



Although not termed a museum, the teaching collec- 

 tions of minerals, rocks, fossils, &c., at the Science 

 Schools are sufficiently full and complete for the most 

 advanced student. Some time was spent in these rooms ; 

 as many of the members of the Association are engaged 

 in teaching science they examined the arrangements with 

 much interest. The elementary collections, which every 

 student is required to know thoroughly, are arranged in 

 table-cases always open to inspection ; the more advanced 

 collections are in drawers beneath. Over the table-cases 

 and drawers which contain the foss'ls there are coloured 

 vertical sections and diagrams of the geological forma- 

 tions and their subdivisions, showing the variations in 

 their development in different districts. 



In the Biological and Geological Lecture-Room an 

 address was deli"vered by Mr. G. A. Cole, Prof. Judd's 

 chief assistant, on " The Preparation of Microscopic 

 Sections of Rocks and Mmerals," illustrated by the appa- 

 ratus employed and by drawings upon the blackboard. 



From the'lecture-room the party passed into the bio- 

 logical laboratory, upon the table of which, for this occa- 

 sion, were placed a large number of microscopes, with 

 sections of rocks and minerals, each with its name 

 attached. From this the visitors passed into the ad- 

 vanced and research laboratories for geology, and thence 

 down the main staircase to the entrance-hall. 



PHOTOGRAPHIC STUDY OF STELLAR 



SPECTRA 



Henry Draper Memorial 



THE study of stellar spectra by means of photography 

 was one of the most important investigations under- 

 taken by the late Prof Henry Draper. He was actively 

 engaged in this research during the last years of his life. 

 His plans included an extensive investigation, one object 

 of which was to catalogue and classify the stars by their 

 spectra. Mrs. Henry Draper has made provision, at the 

 Observatory of Harvard College, for continuing these 

 researches, as a memorial to her husband. The results 

 already obtained, with the aid of an appropriation from 

 the Bache Fund, permit the form of the new investigation 

 to be definitely stated. The part of the sky to be sur- 

 veyed is that extending from the North Pole to the parallel 

 of 30= south declination. Each photograph will be ex- 

 posed for about one hour, and will include a region 10° 

 square. The telescope employed has an aperture of 

 20 centimetres (8 inches\ and a focal length of 117 centi- 

 metres (44 inches). The object-glass is covered by a 

 prism, and the resulting spectrum of each star in the 

 region photographed has a length of about i centimetre ; 

 which enables the character of the spectra of stars from 

 the fifth to the eighth magnitude to be determined. A 

 modification of the apparatus is employed for the brighter 

 stars. 



Meanwhile, experiments are in progress with the 15- 

 inch equatorial, with the object of representing the 

 spectra of some typical stars upon a large scale. The 

 spectra so far obtained are about 6 centimetres in length, 

 and exhibit much well-defined detail. Additional experi- 

 ments will be tried with a spectroscope provided with a 

 slit, as well as with the simple prism hitherto employed, 

 in order to secure the best possible definition. The 

 present results encourage ths expectation that the move- 

 ments of stars in the line of sight may be better detert 

 mined by the photographic method than by direc- 

 observations. 



To keep the astronomical public informed of the 

 progress made in this work, specimens of the photo- 

 graphs obtained will be gratuitously distributed from 

 time to time. The first of these distributions will pro- 



bably be made in a few weeks. Owing to the expense of 

 providing a large number of copies, it is desirable to 

 limit the distribution, as far as possible, to those who are 

 interested in this class of work. It is also desired, how- 

 ever, to send the specimens to all who will find them of 

 value from the scientific point of view. A blank form of 

 request is attached to the present circular, and may be 

 filled out and sent to the Harvard College Observa'ory 

 by any one desirous of receiving the specimens ; but 

 requests to the same eflect in any form which may be 

 convenient will also be cheerfully complied with so far as 

 mav prove practicable. 



Edw.\rd C. Pickering, 

 Director of Harvard College Observatory 

 Cambridge, U.S., March 20 



SOLAR HALO WITH PARHELIA 



ON Thursday, April I, a solar halo with parhelia was 

 seen here, in regard to which, with the consent of 

 the Astronomer-Royal, I beg herewith to offer a few par- 

 ticulars. The best display occurred between ih. 30m. 

 and 2h. p.m., and at one time exhibited the following 

 appearance. There was the large halo commonly seen, in 

 addition to which a luminous ring passing through the 

 sun encircled the s';y, everywhere of the same altitude 

 above the horizon, forming a small circle of the sphere 

 taking the zenith as pole. On this, the parhelic circle, 

 and outside of the halo by about 5°, a mock sun was 

 seen both on the eastern and western sides ; another was 

 seen in about a north-north-west direction, and a fourth 

 nearly east, both also situated on the circle. 



Calling the real sun S, and the several mock suns, 

 counting westward, Si, S., S;„ S4, differences of azimuth 

 were independently estimated as follows : — 



130 



120 



"5 

 120 



By myself, numerical estimation ... 115 



By Mr. Nash, by estimation ... 120 

 By Mr. Lewis, measured froJi a 



sketch 123 ... 115 •■• 122 



Mean II9 ... 122 ... 119 



apparently indicating that the true difference of azimuth 

 was in each case 120'. 



Mr. Turner states that Sj was on the meridian at 

 ih. S5m., at which time the calcidated azimuth of S from 

 south was 36°, which is therefore the difference of azimuth 

 between Sj and S. I estimated this difference to be 31°, 

 Mr. Nash 35°, and Mr. Lewis 35'. Mean^= 34°. This 

 azimuthal measure corresponds to about 27' as measured 

 on a great circle at about the position of the sun. De- 

 ducting 5^ the estimated amount by which Sj or S^ was 

 outside the halo, we have 22° for an approximate value of 

 the radius of the halo, about the usual magnitude. 



The evidence that the altitude of the circle on vyhich 

 the suns were seen was everywhere the same is as 

 follows :— At 2h. the altitude of the sun, by direct calcu- 

 lation, was 37°. At the same time Mr. Turner, by 

 measurement with the transit-circle, found the altitude of 

 the circle at the point at which it crossed the north 

 meridian to be 37', it being well seen ; its altitude on the 

 south meridian appeared to be 40", but the circle at this 

 moment was not distinctly visible at this point. At 

 2h. T5m. Mr. Turner found, with the altazimuth, the alti- 

 tude of both S2 and S3 to be 35°, which, allowing for 

 change of altitude, gives 364^ for the corresponding alti- 

 tude at 2h. 



There were great variations in brilliancy of the different 

 parts during the interval first mentioned, and some of the 

 appearances were visible at a much later hour. The 

 suns S, and S4 at times exhibited prismatic colours in a 

 marked manner. 



