94 Mr. David Gill [May 23, 



provided with a telescope driven by clockwork to follow the diurnal 

 motion of a star, and thus the hands of the observer were free to make 

 the necessary measures. These were accomplished by an instrument, 

 such as I hold in my hands, applied to the telescope. This micro- 

 meter contains two parallel spider-webs each attached to a slide, one 

 slide being moved by one screw, the other by the other screw. The 

 screws are provided with drum-heads divided into 100 parts. One 

 web was placed on the image of a Lyrae, the other upon that of the 

 faint comparison star, and the angle between the stars was thus read 

 off in terms of the number of revolutions and decimals of a revolution 

 of the screws. A number of such observations was made on each 

 night, and the result for each night depended on the mean of the 

 numerous observations made each night. 



By observations on ninety-six nights between November 1835 and 

 August 1838, he showed that the distance between a Lyrae and the 

 faint comparison star changed systematically with a regular annual 

 period, and that the maxima and minima of those distances corre- 

 sponded with the times of the year at which these maxima and 

 minima should occur if the brighter star were really much nearer 

 than the fainter one. 



Assuming that the fainter star is at a practically immeasurable 

 distance, Struve showed that a Lyrae had a parallax that amounted to 

 about a quarter of a second of arc, which is equivalent to the state- 

 ment that a globe whose diameter is equal to that of the earth's orbit 

 — that is, to 186 millions of miles — would at the distance of a Lyrae 

 present an apparent diameter of half a second of arc. If you wish to 

 realise this angle, place a globe 1 foot in diameter at a distance of 

 80 miles, or look at a coin half the diameter of a silver threepenny- 

 piece at a distance of 1 mile from the eye, and try to measure it. 



The great German astronomer, Bessel, was simultaneously 

 en^^aged in like work at Konigsberg. He selected as the object of his 

 researches a very remarkable double star — 61 Cygni. 



This star had already been the subject of similar researches on 

 his part with much inferior means. He now attacked the problem 

 with the splendid heliometer which had been made for him by 

 Frauenhofer for the purpose. The principle of this instrument I 

 shall presently explain. His reasons for choosing 61 Cygni were 

 that the two components of this star, though not remarkable for 

 brightness — they are just visible to the naked eye — yet have this 

 peculiarity, that they have a remarkably large proper motion, the 

 largest then known, though now known to be surpassed by that of 

 two other stars which I shall afterwards mention. The components 

 of 61 Cygni have an apparent angular motion relative to other stars 

 of more than five seconds of arc per annum. 



Struve had argued that if the stars were on the average of similar 

 brightness, those stars which were brightest would probably be those 

 nearest to us, and Bessel, in like manner, argued that if the absolute 

 motions of the stars were similar on the average, those motions which 

 appeared the largest belonged to stars which on the average were 



