168 Mr. David Gill [June 3, 



He has selected for experiment the interesting double star 61 Cygni. 

 One of the original negatives of the series is now on the screen. This 

 star, as is well known, was selected by Bessel, on account of its large 

 proper motion, as the most suitable star for his first experiment. It 

 was probable, because its large apparent motion among the stars was 

 so great, its real distance from us would be less than that of stars of 

 less apparent motion. Bessel's observations with the Konigsberg 

 heliometer proved this to be tbe case, and his discussion of these 

 observations first convinced astronomers that the measui-ement of 

 interstellar spaces was a problem not entirely beyond their reach. 



Prof. Pritchard has now photographed the star during a whole 

 year, and within a few days he promises that we shall have the results 

 of his measurement of the plates. It will be of great interest to 

 comiDare his results with previous independent determinations of tbe 

 parallax of the same star made by other astronomers with difi'erent 

 means, but it will be still more interesting for the futui-e of astronomy 

 to compare the amount of accuracy which the photographic method 

 affords, as compared with the older existing methods. From pre- 

 liminary results published by Prof. Pritchard we are led to expect 

 a very high accuracy from the new process. 



So far, however, as present experience goes, we shall not be able 

 to apply this new method to the measurement of the parallax of very 

 bright stars, because, when the plates have been exposed long enough 

 to obtain pictures of the faint comparison stars, the discs of the 

 brighter stars become too large and ill-defined for exact measurement. 

 It may be that this obstacle will yet be overcome, but at present it 

 has still to be faced. 



On the question of the comparative merits of refractors and re- 

 flectors as the proper instruments for photographic use, very elaborate 

 comparison has been instituted, and much discussion has been 

 held. 



From the simple facts, that the best work yet done has been done 

 in stellar photography by refractors, and that they are in many ways 

 more convenient and simple in use than reflecting telescopes, the 

 Paris Congress unanimously adopted the refractor as the instrument 

 to be adopted for the international star charts. But here is a very 

 remarkable picture taken with the Oxford reflector, which shows star 

 discs very sharp and very round over a very large field of view, viz. 

 eighty minutes of radius. 



In the photography of special objects, such as star clusters and 

 nebulae, much has been done. 



Common's exquisite photograph of the great nebula of Orion you 

 have seen before in this theatre, and for exquisite beauty of detail it 

 has never been excelled. But of this we may be sure that, if 

 Mr. Common is spared in health and strength to complete the great 

 reflector of five feet aperture upon which he is now engaged, that 

 photograph, beautiful as it is, will be far surpassed. Here is 

 another photograph of the same object by Mr. Roberts. So short is 



