136 THE FUNCTION OF LARGE TELESCOPES. 



the low-dispersion spectroscopes used by Duner and Vogel would hardly 

 show them, but they are easily seen with a three-prism spectroscope, 

 and they have been repeatedly photographed with one and with three 

 prisms. Some of these photographs have been measured and the wave 

 lengths of the bright and dark lines determined. A comparison of the 

 results with those obtained for other types of stellar spectra suggests 

 certain interesting relationships, which, if confirmed by subsequent 

 work, will be of service in tracing the course of stellar evolution. 



This is only a single instance of the advantage for stellar spectro- 

 scopic work of the great light-collecting power of large telescopes, 

 but it would be easy to multiply examples. Our knowledge of the 

 peculiar spectra of the stars of the Wolf-Eayet class, all of which are. 

 found in the Milky Way or its branches, is due in large part to the 

 visual and photographic study of these faint objects made by Pro- 

 fessor Campbell with the Lick telescope. In the able hands of Profes- 

 sor Keeler, whose recent election to the directorship of the Lick 

 Observatory is so truly a cause for congratulation, the same powerful 

 instrument rendered possible the determination of the motion in the 

 line of sight of the planetary nebulae. We may well be confident that 

 the future record of the great telescope on Mount Hamilton will be 

 marked by many similar advances. 



I might profitably go on to speak of the advantages of large tele- 

 scopes for the study of the sun, for in no field of research can they be 

 better employed. In photographing the solar faculre with the spectro- 

 heliograph the large image given by a great telescope is particularly 

 useful for purposes of measurement, as well as for the study of the form 

 and distribution of these phenomena. Prominences, too, whether of 

 the quiescent or eruptive class, are best photographed on a large scale* 

 With a large image it may also become possible, under good atmos- 

 pheric conditions, to photograph some of the delicate details in the 

 chromosphere, which, with a small solar image, would be wholly beyond 

 the reach of the photographic method. It is probably in the study of 

 the spectrum of the chromosphere, however, that one best perceives 

 the advantage of a large instrument as compared with a small one. 

 Eecent experience has made this very clearly evident, for with the 

 40 -inch Yerkes telescope it has been possible to see in the chromospheric 

 spectrum a great number of faint bright lines which were wholly beyond 

 the reach of the 12-inch telescope used in my previous investigations. 

 In this way it has been found that carbon vapor exists in the vaporous 

 sea which covers the brilliant surface of the photosphere. 



It will be admitted, I think, from what has been said, that great tel- 

 escopes really have a mission to perform. While, on the one hand, they 

 are not endowed with the almost miraculous gifts which imaginative 

 persons would place to their credit, they do possess properties which 

 render them much superior to smaller instruments and well worth 

 all the expenditure their construction has involved. In answering the 



