Page:The story of the comets.djvu/237

XII. any trace of an impression due to the comet, whereas an objective-prism spectrograph, in which the ratio of focal length to aperture was only 6 to 1, gave a good spectrum in 30 minutes. Frost's dictum is that "the great refractor, with its long focal length, is obviously unsuited for such an object".

It is obvious then, that if our knowledge of cometary spectra is to be substantially increased by the appearance of Halley's Comet, or by any other bright comet that may be discovered, the number of these short-focus prismatic cameras and specially designed spectroscopes must be multiplied. By using them while the comets are faint, and then working them in conjunction with the larger instruments—giving more precise results—as the comets become brighter we may be able to throw more light on any spectral changes which take place as the comets approach their perihelia. It is probably behind these spectral changes that the truth lies. That all cometary spectra, at all times, are not exactly alike may now be accepted as proven; and it must be borne in mind that it is from the variations between one comet and another, or in the spectra of the same comet at various times, that we are likely to learn most.

After this digression we will proceed to review the work done during the interval since 1882, which has been an interval marked by a large number of small comets.

Borelly's Comet of 1890 (i.) showed on Jan. 15, according to Backhouse, the three characteristic bands with a faint continuous spectrum, the latter being so faint four days later, on January 19, that its presence was only suspected.

Fowler compared the spectrum of Comet 1890 (ii.) (Brooks's) with the spectrum of the blue base of a spirit-lamp flame, and found that the bands coincided, with the exception of the bright fluting in the violet; the continuous spectrum extended from D to a little beyond the band at wave-length 474.

Examining the spectrum of Swift's Comet (1892. i.), Konkoly found five lines which he gave in the following