406 Mr. David Gill [May 29, 



the Cape, I acquired the instrument from Lord Crawford, and carried 

 out certain researches with it on the distances of the fixed stars. 



In 1887, when the Admiralty provided the new heliometer for 

 the Cape Observatory, this instrument again changed hands. It 

 became the property of Lord McLaren. I felt rather disloyal in 

 parting with so old a friend. We had spent so many happy hours 

 together, we had shared a good many anxieties together, and we knew 

 each other's iceaknesses so well. But my old friend has fallen into good 

 hands, and has found another sphere of work. 



The principle of the instrument is as follows. [The instrument 

 was here explained.] 



There is now on the screen a picture of the new heliometer of 

 the Cape Observatory, which was mounted in 1887, and has been in 

 constant use ever since. It is an instrument of the most refined 

 modern construction, and is probably the finest apparatus for refined 

 measurement of celestial angles in the world. 



[Here were explained the various parts of the instrument in 

 relation to the model, and the actual processes of observation were 

 illustrated by the images of artificial stars projected on a screen.] 



Here, again, there is little that conforms to the popular idea of an 

 astronomer's work ; there is no searching for objects, no contemplative 

 watching, nothing sensational of any kind. On the contrary, every 

 detail of his work has been previously arranged and calculated before- 

 hand, and the prospect that lies before him in his night's work is 

 simply more or less of a struggle with the difficulties which are 

 created by the agitation of the star images, caused by irregularities 

 in the atmospheric refraction. It is not upon one night in a 

 hundred that the images of stars are perfectly tranquil. You have 

 the same effect in an exaggerated way when looking across a bog on 

 a hot day. Thus, generally, as the images are approached, they 

 appear to cross and recross each other, and the observer must either 

 seize a moment of comparative tranquillity to make his definitive 

 bisection, or he may arrive at it by gradual approximations till he 

 finds that the vibrating images of the two stars seem to pass each 

 other as often to one side as to the other. So soon as such a bisec- 

 tion has been made the time is recorded on the chronograph, then the 

 scales are pointed on and printed off, and so the work goes on, varied 

 only by reversals of the segments and of the position circle. 

 Generally, I now arrange for 32 such bisections, and these occupy 

 about an hour and a half. By that time one has had about enough of 

 it, the nerves are somewhat tired, so are the muscles of the back of 

 the neck, and, if the observer is wise, and wishes to do his best work, 

 he goes to bed early and gets up again at two or three o'clock in the 

 morning, and goes through a similar piece of work. In fact this 

 must be his regular routine night after night, whenever the weather 

 is clear, if he is engaged, as I have been, on a large programme of 

 work on the parallaxes of the fixed stars, or on observations to deter- 

 mine the distance of the sun by observations of minor planets. 



