THE DISTANCES OF THE STARS. 



295 



Bessel found the distance of 61 Oygni, a star which is not very 

 bright, but which was known to be moving rapidly across the 

 sky, and therefore presumably near. In many ways the most 

 interesting of these observations was that made by Bessel, 

 because he devised a specially delicate instrument, which was 

 very suitable for these refined measurements. This instrument, 

 called a heliometer, was used with marked success by other 

 astronomers, and notably by Sir J3avid Gill, a former Hon. 

 Correspondent of the Victoria Institute. It would be out of place 

 for me to enter into the numerous precautions which have to be 

 taken if reliable results are to be obtained. Industry and skill 

 and a real genius for avoidmg the many errors which instruments 

 are heir to, must be combined in the person of one astronomer. 

 Perhaps I may tell you a story about Sir David Gill. He had 

 been lecturing on this subject, and in order to explain the small 

 angles we had to measure, compared them to the angle which a 

 threepenny bit would subtend at the distance of a mile. A 

 brother Scot, in proposing a vote of thanks to the lecturer, said 

 there could be no doubt of his nationality, for no one but a 

 Scotsman would take any notice of a threepenny bit a mile 

 distant. 



A great simplification in measuring the distances of stars was 

 brought about by the introduction of photography to astrono- 

 mical observations, but it must not be supposed that the task is 

 easy : great care is necessary to avoid small errors which would 

 vitiate the results. Nevertheless, there are at the present time 

 seven or eight observatories with large photographic telescopes 

 where this work is successfully carried on. It is quite possible 

 with a dozen good photographs taken at suitable times to measure 

 the distance of a star if it is nearer to us than 5 million times 

 the sun's distance — that is to say, between 400 and 500 million 

 million miles away from us. 



I have gone into this at length because it seems to me 

 important to give an idea of the methods employed, as well as 

 of the results obtained. The principle underlying the method 

 is simplicity itself, but the successful application of the prin- 

 ciple has been beset by many difficulties. The measurement of 

 these small angles has been made possible by the genius of the 

 engineers who have designed and executed the delicate move- 

 ments of the telescopes, the opticians who have made the large 

 and perfect lenses, and the chemists who have shown us how to 

 obtain by photography a permanent impression of the light sent 

 us by the stars. In these different ways our human faculties 

 have been so greatly extended that we are able to measure 



