Memoirs of the Royal Astronomical Society Volume 50-51 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1892 Excerpt: ...through the prism without the aid of nn ocular. Its spectrum then appears as a fairly well defined circular disc cut off by one or both edges of the prism; and this image indicates the adopted adjustment when its segments are equal. An error of adjustment greatly exceeding any probable misjudgment of the equality of the segments--viz. when one of them is reduced to nothing--gives an error in wave-length of three tenth-metres in the region of G, when F is set to its position on the scale; so that with moderate care in setting the instrument there is little room for erroneous measurements. But it was not until after the spectrum of the Nova had been photographed that my attention was drawn, while constructing the wave-length curve, to the necessity of care in this adjustment. And when the plates were exposed on the night of February 3 some difficulty had been experienced in adjusting the spectrograph on account of the feeble light of the star and the glare of the Moon. It became, therefore, necessary to discover, if possible, what was the actual adjustment for the incident rays on that night; and for this purpose a plate was exposed to a Lyrce with the two extreme errors of adjustment, already mentioned, successively. The difference of the scale measures from F to G on the two spectra of this plate afforded the means of ascertaining the extreme wave-length error quoted above; and when the plate was compared with that of the Nova by placing their films and edges together, the line F of the latter was found to be midway between the two positions of F in the former. This was considered satisfactory evidence of the correct adjustment of the instrument for the Nova; but the hydrogen lines at G' and h appeared out of their places on the map, and to bring them right...