Researches in Stellar Photography. 



205 



■opportunity to make the whole subject the basis of a further communi- 

 cation. I will, however, add here that I have found the fringes of 

 discrete dots which surround the images on the original negative 

 ^entirely disappear on transferring them to a positive print, and leave 

 their positive images simply as clean cut disks, whose diameters are 

 .■sensibly the same as the measured diameters of the concrete disks on 

 the original negative. It is conceivable that in printing, the light 

 penetrates through the interstices of the fringe. 



III. — Applicability of Photography to Astronomical Measurements of 



Great Precision, 



The scientific interest and importance attached to the possession of 

 a register of the relative lustre of the stars, virtually exhibited on a 

 permanent photographic film, capable of re- examination at any time, 

 and accessible at will, are, if possible, surpassed by the value attach- 

 able to the measurability of their relative positions, provided only the 

 accuracy of the measures made upon the film is equal to that obtain- 

 able by the application of the best astronomical instruments to the 

 sky itself. Three questions, in fact, here present themselves in 

 relation to this matter. The first is, whether the photographic 

 pictures accurately represent a portion of the skies ; the second is, 

 whether the relative co-ordinates of the star images are measurable 

 with as much delicacy and certainty as they are by the heliometer, or 

 any other form of micrometer, applied to the sky itself: and the 

 third question has reference to the permanence of the photographic 

 film. 



For present convenience, I shall consider the reply to the first two 

 of these questions as involved in the same evidence. In order to 

 settle these points, several photographic plates of stars situated in 

 portions of the Pleiades group, already referred to, were selected for 

 examination. On each of these the image of rj Tauri (Alcyone) is 

 impressed, together with that of many other stars in the group, and 

 specially for this purpose twenty-five of the stars, whose distances 

 from Alcyone had been so carefully measured by Bessel with his 

 famous heliometer in 1838 — 1842. These distances from Alcyone on 

 the several films were each measured, and as far as possible with 

 as many repetitions as those adopted by Bessel himself. Inasmuch as 

 the plates were taken at different altitudes on different nights, the 

 necessity arose for applying small numerical corrections for the 

 varying effects of refraction, and where necessary, other small correc- 

 tions for aberration were applied, in order that the resulting dis- 

 tances might be strictly comparable. inter se. This examination of the 

 amount of correspondence in the measures is the only object here, for 

 the moment, entertained. 



VOL. XLI. P 



