416 Progress in Science. [July, 



formed by the clock that the image of each star is a clear round dot, without the 

 slightest trace of elongation in the direction of the star's movement. There 

 are about thirty stars registered in the photograph, clustered round Alcyone, 

 the chief of the group ; and it is very remarkable that the several stars are 

 actually of different magnitudes, from Alcyone, which looks very much like a 

 period of punctuation in printer's diamond type, to vanishing dots that can 

 only be discerned by the help of a lens. The magnitude of the stellar disc is 

 most probably a result of diffraction ; but however this may be, there certainly 

 it is. The stars on the photographic plate are not mathematical points, but 

 little dots of different sizes, just according to the notion that the eye forms of 

 them in looking on the group with an aperture of large diameter. There is one 

 very ingenious piece of precautionary contrivance connected with the photo- 

 graphic impression of these stars well worthy of note. Mr. Rutherford's main 

 object, in the labour he has been incurring in perfecting his process of celestial 

 photography, is to afford a ready means of providing registers of the exact 

 positions of the fixed stars which shall be altogether free from the possible 

 errors of personal observation. Now it unfortunately happens that photo- 

 graphy has an awkward and mischievous trick of making stars on its own 

 account, which may readily be confounded with the photographic impressions 

 of the proper host of heaven. Any speck of accidental imperfection in a photo- 

 graph may be readily taken for the image of a star. It therefore becomes a 

 matter of paramount necessity, when groups of fixed stars are to be dealt with 

 for astronomical purposes, that some method shall be devised whereby the 

 portraits of the stars shall be distinguished from the photographic accidents 

 and star-ghosts. This object Mr. Rutherford has most efficiently and satis- 

 factorily accomplished by the simple expedient of covering the object-glass of 

 the telescope, and disconnecting the instrument from the clock-movement for 

 a few seconds, after the six minutes' exposure necessary to give the image of 

 a fixed star, and then again attaching the telescope to the clock and giving a 

 second exposure of six minutes. The immediate effect of this is that every 

 true star image is closely followed by its double — thus . . ; in other words, 

 every star photographically pourtrayed is a double star ; and this process of 

 re-duplication is obviously one which photographic imperfection, or accident, is 

 quite unable to simulate. When once the photographic images of a group of 

 fixed stars are impressed upon a plate, the measurement of their relative dis- 

 tances becomes a mere matter of mechanical operation. But in Mr. Ruther- 

 ford's proceeding the relative positions of the stars is also fixed by securing 

 upon the photographic plate a tracing of the direction in which some star 

 moves across the plate, when the telescope is not carried by the clock-move- 

 ments — that is, a tracing of a line coinciding with a circle of declination. 

 Bright stars, such as Sirius and Vega, leave a distinct trail as they travel 

 across the plate. In the case of fainter stars the same result is secured by 

 giving a second exposure of the star after a brief interval. A line then drawn 

 from the first image of the star to its second image gives the exact direction of 

 the declination parallel. In the photograph of the Pleiades it will be seen 

 that Alcyone has thus been made to leave a position-impression nearly half 

 way across the plate from the re-duplicated image first registered, and that 

 these successive images have been subsequently connected, for purposes of 

 reference and measurements, by a visible pencil-line. Three large telescopes 

 have already had their object-glasses corrected for photographic work under 

 this plan of Mr. Rutherford's. The first on which the correction was made was 

 composed simply of two discs of glass constructed under the ordinary arrange- 

 ments of the achromatic combination, and had an effective aperture of 

 xi\ inches. A second, with an effective aperture of 13 inches, is now in use 

 in Mr. Rutherford's observatory; and this large instrument has ingeniously 

 been fitted to be used for ordinary astronomical vision, or for photographic 

 work, by applying the photographic correction through the instrumentality of 

 a separate constituent. The object-glass has such a figure that it is visually 

 perfect, until it is thrown unto photographic equipment by the addition to it of 

 a third lens of glass. It is primarily composed of the ordinary double lens, 

 one of flint- and one of crown-glass. The photographic correction is all con- 



