A little ramble through 19th century astronomy - Stars, Saturn and photographs

 Academy, Saturday 19^th June 1875

 

Royal Astronomical Society (Friday, June 11). Professor Adams, President, in the Chair.

A paper by Mr. Knobel was read, giving some results of measures of magnitudes of stars with his astrometer, described in a previous paper, the principle of the instrument being to reduce the aperture of the telescope by means of a variable triangular diaphragm till the star disappeared. Mr.Knobel  pointed out several discordances between his results and the magnitudes given by Argelander in his Uranometria, though those of the Bonn Durchmusterung agreed better.

 Mr, Marth called attention to approaching phenomena of the satellites of Saturn, the most important of which, however (those of Titan), would only be visible in Australia and in the United States, the period of revolution being very nearly sixteen sidereal days, so that occultations and transits of this satellite would take place at the same sidereal hour for many successive periods, and at these times Saturn would be below our horizon. Mr. Marth expressed a hope that the great Melbourne reflector might be used for these observations, and Mr. Russell promised that he would employ the 11-inch refractor of the Sydney Observatory for this purpose.

 Captain Abney, R.E., gave a description of his Diaphanometer, an instrument which he had devised primarily for measuring the opacity of photographic films after exposure to light under different conditions, and which he had since applied to other questions in photometry. In this instrument the collodion film was compared with a graduated wedge of smoke-coloured glass. Captain Abney also exhibited an ingenious form of spectroscope in which the brightness of any part of the spectrum of a star could be compared with that of the spectrum of a standard source of light, the two spectra being brought one above the other in the same field by means of reflection prisms.

Some other short papers of a purely technical character followed, after which a note by Mr. Proctor on “ Photography in the Transit of Venus” was read, to which Mr. Russell, Mr. De La Rue, and Mr. Christie replied, and Mr. Neison mentioned, as bearing on the question of the atmosphere of Venus (which  Mr Proctor had incidentally  referred to), that Professor Lymans had observed Venus as a bright ring five hours before the commencement of the late transit.

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