February 21, 1895] 



NA TUR£ 



597 



tations of ihe heavenly bodies have been produced ; these I 

 shall have to show you at one time or another in relation to 

 different branches of our subject. In this instrument (Fig. 5) a 

 reflecting telescope of 20 inches aperture is combined with a 

 refractor of 7 inches aperture. The refractor is used as a 

 guiding telescope, and ensures that the imasjes of the stars and 

 tiebuKc fall on the same part of the photographic plate which 

 is being exposed in the refleding telescope throughout the whole 



I I.,, :, — I'l. Roberts' twin telescope. 



lime of the e.vposure. Even with the best driving clocks, such 

 a guiding telescope cannot be dispensed with when the ex- 

 posures are prolonged for the number of hours necessary in some 

 cases. 



• i First for the nebulae. What is the difference, written down 

 in this way, between a nebula and a star cluster? A com- 

 parison of Figs. 6 and 7 will at once show that in the case of 

 a star cluster we have to deal with a collection of separate 

 stars, while in the case of a nebula there is a filmy sort of 



Fig. t. — The Cluster 15 M Libr.-r, from .1 phol o'r.-iph hy Or. Kotirrts. 



luminosity which is quite distinct from the neighbouring stars. 

 Here and there the nebulosity is suddenly brightened, but, as 

 I shall show hereafter, these condensations are not to be 

 regarded as stars in the ordinary sense. Here is Ihe 

 nebula of Orion, which we owe to th.it wonderful telescope of 

 Dr. Roberts. Several of you may have seen the nebula of 

 Orion with a telescope, but you have never seen it exactly like 

 this. Vou get here the idea which gave rise to the old notion 

 of a candle shining through horn ; this nebula is the one which, 



NO. 132 I, VOL. 51I 



on account of its brightness, spectroscopically gives us most 

 easily indications of those bright lines which for ever .<et at rest 

 the idea that we are dealing with solid or liquid bodies. 



••Vt the beginning of the present century it was found that in 



Fig. 7. — The spiral nebula in Canes Venatici, from a photograph by 

 Dr. Roberts. 



order to get the spectra of stars the best thing to do was to 

 put a prism outside the telescope, and to let the light enter 

 the telescope and be brought to a focus after it had passed 

 through the prism ; and it is a most unfortunate thing, that the 



ing nielhi '1 <it p': 



prism. 



neglect o( the application of this principle has lan'Jed us pro- 

 bably in a delay of fifteen or twenty yerrs in g.ilhering know- 

 ledge on I I>is subject. The whole credit of reviving this idea 

 is due to Prof. Pickering, of'the Harvard College, who since 



