132 THE FUNCTION OF LARGE TELESCOPES. 



seen with the larger one. But with better atmospheric conditions, to 

 my eye at least, the advantage lies wholly on the side of the larger 

 instrument, whether the object be the moon, Jupiter, Mars, or Saturn. 

 In the case of the moon particularly, much fine detail which I have 

 never been able to see with the 12-inch telescope is clearly and beauti- 

 fully visible with the 40-inch. I am certainly inclined to think that 

 large telescopes are greatly to be preferred to small ones for work of 

 this character. But I give much less weight to my own opinion on this 

 subject than to that of Professor Barnard, who for many years has 

 observed the planets with instruments varying in size from a 5-inch 

 telescope to the 36-inch on Mount Hamilton, and the 40-inch of the 

 Yerkes Observatory. He believes a large aperture to be immeasurably 

 superior to a small one for these observations. This seems to me quite 

 sufficient to settle the question, for it would be difficult to name a bet- 

 ter authority. 



One incidental advantage of such an instrument as the 40-inch tele- 

 scope, which depends to a great degree upon the stability of its mount- 

 ing, is the ease and certainty with which micrometrical measures can 

 be effected. Since the telescope was first ready for regular use last 

 September, Professor Barnard has made with it a long series of micro- 

 metrical measures, which have included such objects as the satellite of 

 Neptune, the companion to Procyon, and the fifth satellite of Jupiter. 

 The precision of these measures is most satisfactory, and lends special 

 interest to an attempt which he has made to determine the parallax of 

 the nebula N. G. C. 404, which is in the field with the bright star /3 

 Andromeda}. This object has a definite condensation, which permits 

 its position to be accurately determined with reference to a number of 

 stars in the neighborhood. A long series of measures, covering a period 

 of five months, have led to the conclusion that the nebula can not pos- 

 sess a parallax as great as half a second of arc, and, therefore, can not 

 be nearer the earth than about four hundred thousand times the distance 

 from the earth to the sun. 



Mention should be made of one more interesting observation by Pro- 

 fessor Barnard, which would have been much more difficult with a small 

 telescope. It will be remembered that in the valuable work which Pro- 

 fessor Bailey has been doing at the station of the Harvard College 

 Observatory in Arequipa, Peru, excellent photographs were obtained 

 of southern star clusters, which show that these clusters contain an 

 extraordinary number of variable stars. Not only do scores of stars in 

 a single cluster vary in their light, but the change is exceedingly rapid, 

 occupying in some instances only a few hours. So far as I know, none 

 of these remarkable variations had been seen visually until Professor 

 Barnard undertook the systematic observation of one of the clusters 

 with the 40-inch telescope. On account of the large scale of the images, 

 he is able to distinctly see stars in the cluster without confusing them 

 with others in their neighborhood, and has thus been enabled to follow 



