NOTICES AND ABSTRACTS 



OF 



MISCELLANEOUS COMMUNICATIONS TO THE SECTIONS. 



MATHEMATICS AND PHYSICS. 



Address by Warren De La Hue, Esq., D.C.L., Ph.D., F.R.S., V.P.C.S., 

 V.F.R.A.S., President of the Section. 



My predecessors in this Cliair liave addressed you on many subjects of high interest 

 in Mathematical and Physical Science : I do not contemplate passing in review 

 the recent discoveries in Astronomy or Physical Science, but intend to confine 

 myself, in the main, to Astronomical Photography; and in selecting this br<anch of 

 science as the subject of this introductory discourse, 1 think that I shall have your 

 approval, not only because I have given special attention to that subject, but also 

 because it is about to be applied to the determination of a fundamental element 

 of our system, the solar parallax, by observations of the transit of Venus in 1874, 

 and probably also in 188:^. 



Nothing is so lastingly injurious to the progress of science as false data; for 

 they endure often through many centuries. False views, even if supported b}^ some 

 amount of evidence, do comparatively little harm ; for every one takes a salutary 

 interest in proving their falseness ; and when this is done the path to error is 

 closed, and the road to trutli is opened at the same moment. 



It will be conceded that Photography, when applied to scientific observation, un- 

 doubtedly preserves facts. But the question has sometimes been raised, are plioto- 

 graphic records absolutely trustworthy representations of the phenomena recorded ? 

 If not, what is the extent of truth, and where are the inlets for errors and mistakes ? 

 Not only has photographic observation gained a wide range of applications in astro- 

 nomy, but in ever)' other branch of phj-sical science its help is daily more and more 

 taken advantage of; and although, in speaking of this art, I shall confine myself to 

 astronomy, the obser\ations which I propose to make may be suggestive with 

 reference to other branches of phj'sics. 



As an instance of the application of this art to optical physics I may in this place 

 call attention to the very successful delineation of the solar spectrum by Mr. Lewis 

 M. Rutherfurd, of the United States. In Mr. Rutherfurd's spectrum, obtained by 

 the camera, many portions and lines are shown (in the ultra-violet for instance) 

 which, while imperceptible to the retina of the^ eye, impress themselves on the 

 sensitive film. As a fact, lines which are single in Angstrom's and Kirclihofl''s maps, 

 have been recorded by photography as well-marked double lines. I will now review 

 the application of the art to astronomy. 



Stellar photography was for some time applied at Harvard-College Observatory, 

 U.S., to double stare, for the purpose of determining by micrometric measurement 

 their relative angle of position and distance. The zero of the angle of position was 

 found by moving the telescope in right ascension after an impression had been 

 taken, and taking a second one on the same plate ; this process gave two sets of 

 photographic images on the same plate ; and the right line passing through the 



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