1871J Light. 415 



and by direct comparison with the spectra of terrestrial substances. The 

 spectroscope was furnished with one prism of dense flint-glass, having a 

 refracting-angle of 6o°, an observing telescope magnifying 5^ diameters, and a 

 collimator of 5 inches focal length. A cylindrical lens was used to increase 

 the breadth of the spectrum. The remarkable absorption taking place at 

 Uranus shows itself in six strong lines, which are drawn in the diagram. 

 The least refrangible of these lines occurs in a faint part of the spectrum, and 

 could not be measured. Its position was estimated only, and on this account 

 it is represented in the diagram by a dotted line. The positions of the other 

 lines were obtained by micrometrical measures on different nights. The 

 strongest of the lines is that which has a wave-length of about 544 millionths 

 of a millimetre. The band at 572 of the scale is nearly as broad but not 

 so dark; the one a little less refrangible than d is narrower than the others. 

 The measures taken of the most refrangible band showed that it was at or 

 very near the position of f in the solar spectrum. The light from a tube con- 

 taining rarefied hydrogen, rendered luminous by the induction-spark, was then 

 compared directly with that of Uranus. The band in the planet's spectrum 

 appeared to be coincident with the bright line of hydrogen. There is no 

 absorption-band at the position of the line of sodium. It will be seen by 

 a reference to the diagram that there are no lines in the spectrum of Uranus 

 at the positions of the principal groups produced by the absorption of the 

 earth's atmosphere. On April 7th a faint comet was discovered by Dr. Win- 

 necke. Dr. Huggins observed the comet on April 13 and May 2. It pre- 

 sented the appearance of a small faint coma, with an extension in the 

 direction from the sun. When observed in the spectroscope, the light of the 

 coma was seen to consist almost entirely of three bright bands. A fair 

 measure was obtained of the centre of the middle band, which was the 

 brightest; it gives for this band a wave-length of about 510 millionths of a 

 millimetre. The position of the less refrangible band was only estimated 

 roughly. The result gives 545 millionths. The third band was situated 

 at about the same distance from the middle band on the more refrangible side. 

 It would appear that this comet is similar in constitution to the comets which 

 Dr. Huggins examined in 1868. 



Mr. Rutherford has recently communicated to the Royal Astronomical 

 Society a full description of the apparatus and arrangements he has devised 

 for producing photographs of the sun and fixed stars ; and at the last meeting 

 of the Photographic Society Dr. Mann made a communication on the same 

 subject, from which we condense the following particulars : — Mr. Rutherford's 

 success has been mainly due to the photographic excellence of his telescopes 

 — the object-glasses of his instruments having been corrected for photographic 

 work, with peculiar care, under a plan of his own. The instruments, in their 

 photographic equipment, are simply valueless for all ordinary purposes of 

 astronomical vision, but they are as nearly perfect as may be for astronomical 

 photography. The object-glasses of all astronomical telescopes are carefully 

 corrected both for chromatic dispersion and for spherical aberration. But 

 these corrections require a very material re-discussion and modification when 

 the object-glasses are fitted to perform the highest class of photographic work. 

 This further modification, in the case of his instruments, Mr. Rutherford has 

 mainly accomplished by a very elaborate and refined process of step-by-step 

 observation of imperfections in the photographic image, and step-by-step 

 correction of the imperfections as they are noted in the successive observa- 

 tions. The photograph of the sun's disc presented to the Royal Astronomical 

 Society is of exquisitely fine definition. The faculse are clearly rendered ; and 

 a large group of dark spots are shown in their minutest detail, with the inci- 

 pient bridging across one of them in the process of approaching disruption. 

 The photograph of the Pleiades, however, is a much more remarkable object. 

 This photograph was made with a telescope of 13 inches aperture, with six 

 minutes' exposure, the telescope being, of course, carried during the time of 

 exposure by a very carefully adjusted clock, so as to keep the image of each 

 individual star upon exactly the same point of the sensitive plate during the 

 whole period of exposure; and this service has been so marvellously well per- 



