Page:The story of the comets.djvu/236

182 was estimated to be about  or  the distance between the D lines, a displacement which would correspond to a motion of about 42 miles per second. The actual motion of recession of the comet had been otherwise put at about 45 miles per second.

As the comet receded from the Sun the various bright lines faded away, the D lines lasting longer than the others, whilst the carbon bands came into greater prominence. There is every reason to believe that these changes are strictly typical; and that we may expect comets of short perihelion distance to show, first, the hydro-carbon spectrum; then, when in the neighbourhood of the Sun, the sodium lines; and lastly, when at closest approach, a number of other metallic lines, and particularly those of iron. Lockyer carries this view of the successive changes of cometary spectra much further, and has drawn up a long list of the successive phases presented by the spectrum of a comet as the body approaches or recedes from the Sun.

Since 1882 there has been no "great" comet, but improvements in the methods of observation and an increased interest in the physical conditions of these mysterious objects are factors in the case which have more than counterbalanced the lack of bright comets available for investigation. The increased sensitiveness and reliability of the photographic plates in use have also played an important part in the investigations by bringing to light the ultra-violet radiations in which cometary spectra are so rich and to which the eye is blind.

It has already been remarked that the earlier photographic investigations of cometary spectra were made with instruments designed for stellar researches only and unfitted for cometary work. Thus Professor Frost records that, on Oct. 30, 1908, an exposure of 158 minutes with the 40-inch Yerkes refractor and the one-prism Bruce spectrograph failed to give