Page:The story of the comets.djvu/228



spectroscope has not been employed in the study of comets on the same scale as it has been used for the purposes of solar and stellar research, and consequently the harvest of knowledge obtained by it has not been so great as one could have wished. Until quite recently most cometary spectra observations were made visually, thus excluding the violet and ultra-violet radiations. Prior to 1902 all attempts to photograph the spectra had been made with long-focus telescopes and spectroscopes, primarily designed for stellar work, a procedure now very generally recognised as inadequate except in the case of an exceptionally bright comet.

A comet is a diffuse object, and is not, usually, very bright. To obtain a spectrum of such an object it is essential that the light be condensed as much as possible; therefore long- focus telescopes and spectroscopes which spread out the light are unsuitable for cometary work. For this reason short-focus prismatic cameras have recently been employed, and