June 5, 1884] 



NA TURE 



137 



of London should acknowledge the labours of him who had most 

 effectually contributed to this end. 



Henderson's results seemed sufficiently convincing, but they 

 depended upon determinations of the absolute place of a Cen- 

 tauri. The experiences of the skilful astronomer Brinkley at 

 Dublin were still fresh in the minds of astronomers. He had 

 arrived by similar though less perfect means at results like those 

 of Henderson ; but his results had been proved to be fallacious, 

 though the causes of their being so still remain somewhat inex- 

 plicable. In the case of Struve's observations the weight of evi- 

 dence which he produced and the excellence of his method were 

 admitted, but men were not prepared by experience for accept- 

 ing as accurate the minute changes of angle which Struve had 

 to measure — nor, I am bound to admit, was the proof afforded 

 by Struve's series of observations so entirely convincing as that 

 afforded by the series of Bessel. Therefore to Bessel the well- 

 earned medal was given, but the labours of Struve and Henderson 

 received high and honourable mention. I quote from the speech 

 of Sir John Herschel in awarding that medal. He says of 

 Henderson's researches on a Centauri : — 



" Should a different eye and a different circle continue to give 

 the same result, we must of course acquiesce in the conclusion ; 

 and the distinct and entire merit of the first discovery of the 

 parallax of a fixed star will rest indisputably with Mr. Henderson. 

 At present, however, we should not be justified in anticipating a 

 decision which time alone can stamp with the seal of absolute 

 authority. " 



So much for Sir John Herschel's officially expressed opinion. 

 I can state now, and as Henderson's successor I do so with pride 

 and pleasure, that a different eye (that of his able and sympa- 

 thetic successor, Sir Thomas Maclear) fully confirmed Hender- 

 son's result with another circle ; and further, that Henderson's 

 result has been still further confirmed by additional researches of 

 which I shall presently speak. 



I must now pass over briefly the history of succeeding 

 researches, and indeed it has been so admirably and so recently 

 told within these walls by Dr. Ball that it is quite unnecessary I 

 should enter upon it in detail. The most reliable values arrived 

 at for the parallaxes of the stars of the northern hemisphere are 

 given in the following table, and to these results I shall after- 

 wards refer : — 



TABLE I. — Parallaxes of Stars which have been determined in 

 the Northern Heavens with considerable Accuracy 



The recent researches referred to in the title of this evening's 

 lecture are some investigations which, in conjunction with a 

 young American friend, Dr. Elkin, who was my guest for two 

 years, I have recently carried out at the Cape of Good Hope. 



The instrument employed was a heliometer — my own property 

 — the good qualities of which I had previously tested at Mau- 

 ritius in 1874 and at the Island of Ascension in 1877. 



Now what is a heliometer ? It is a telescope of which the 



object-glass is divided thus h J , and the two segments so formed 

 can be moved with respect to each other, thus L . 1 , and <^ _ 



Here is a model which has been constructed to illustrate the 

 principle of the instrument. You see that when the two segments 

 are brought into what we may call their natural position, thus 

 t — 4, that a heliometer differs in no way from an ordinary tele- 

 scope — its divided lens produces a single image of a point of 



light, as will be evident from the image of the single artificial 

 disk now on the screen. In optical language, the optical centres 

 of the two segments are in coincidence, and so the images pro- 

 duced by each segment of the lens are in coincidence. But now, 

 if the segments are separated, either segment produces a separate 

 image of the artificial star, and the separation of the images is 

 proportional to the separation of the segments. 



Now, to illustrate how this instrument is used in observation, 

 let there be two artificial stars — a and b. When the optical 

 centres of the segments are in coincidence, we have on the screen 

 — or in the field of view of the telescope — the images of these 

 two stars. By separating the optical centres of the segments thus 

 lj\ we obtain double images of each of the stars a and b. Now 



if we turn the direction of the line of motion of the divided seg- 

 ments parallel to the direction of the stars a and b, and if we 

 separate the lenses sufficiently we can make one of the images of 

 the star a coincide with one of the images of star b. Similarly 

 if we cross the segments we can bring the second image of star b 

 into coincidence with the second image of star a, and if we have 

 finely divided scales attached to the slides by which the seg- 

 ments are separated we can read off, in terms of these scales, the 

 amount of this separation, and this separation is obviously twice 

 the angle between the stars a b. 



There is now upon the screen a photograph from a drawing 

 illustrating the arrangements by which the segments of my helio- 

 meter are moved, and showing the scales by which the amount 

 of the movement is measured ; and these scales are read off by 

 a powerful microscope from the eye end of the telescope, as in 

 the photograph of the instrument now on the screen. 



There is now on the screen a photograph of a drawing of the 

 most perfect heliometer in the world, recently made by Messrs. 

 Repsold of Hamburg for the Observatory of Yale College, 

 New Haven, U.S. That instrument is now under the charge of 

 my young friend, Dr. Elkin, of whom I have already spoken. 

 If then we wish to observe the angle between two stars, it is 

 only necessary to separate the segments of the object-glass by 

 the required amount, to rotate the tube till the line of section of 

 the object-glass is in the line joining the stars, to direct the axis 

 of the telescope to a point in the heavens midway between the 

 two stars under observation, and then we shall find in the field 

 of view the two stars the angle between which we wish to 

 measure. Then by slow and delicate changes in the distance of 

 the optical centres of the segments, whilst the images of the 

 stars are made to pass and repass through each other — thus — we 

 are able to exactly adjust the angular distance of the segments to 

 correspond truly with angular distance of the stars. 

 ( To be continued. ) 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 



Cambridge. — The Museums and Lecture-Rooms Syndicate 

 have recommended the immediate erection of a new lecture- 

 room for physiology, with large additions to the rooms for 

 practical physiology and to the work-rooms adjoining the Com- 

 parative Anatomy Museum, at an estimated cost of 7500/. 

 These are to be carried along Corn Exchange Street. A work- 

 room for the large class of Elementary Biology is also recom- 

 mended to be built as an additional story above the Museum of 

 Mineralogy, at a cost of about 2500/. It is also recommended 

 that 1000/. be laid out in the purchase of microscopes. 



The Board of Biology and Geology have modified their report 

 respecting demonstrators and lecturers in Animal Moiphology, 

 on learning that the General Board of Studies cannot support 

 their former proposals owing to the financial state of the Uni- 

 versity. They now ask for a lecturer on Vertebrates at 100/. 

 and one on Invertebrates at 50/., together with a demonstrator 

 at 150/., to be appointed by the Senior Lecturer in Animal 

 Morphology. 



The Rev. J. Venn, of Gonville and Caius College, has] been 

 approved for the degree of D. Sc. 



SOCIETIES AND ACADEMIES 



London 

 Royal Society, May 15. — " Some Experiments on Metallic 

 Reflection. No. V. On the Amount of Light Reflected by 

 Metallic Surfaces. III." By Sir John Conroy, Bart., M.A. 

 Communicated by Prof. G. G. Stokes, Sec.R.S. 



