THE MEASURE OF STELLAR SPACES 333 



of Sirius. It followed from this that if they are the same in 

 size or light-giving power, Sirius is at least 27,000 times the 

 distance of the sun. 



The computation was vague ; it was, after all, an estimate 

 and not an exact measure, and it was an exact measure which, 

 with the splendid advance of the science in other fields, now 

 became the goal of stellar observers. 



About the time that Bradley, despairing of any direct de- 

 termination, had turned his attention from questions of parallax 

 to that of the proper motion of the stars, an idea occurred ap- 

 parently to several minds which was to bring the solution of 

 the problem. One of these was a young Englishman named 

 Savery. fated, like Gascoigne, his predecessor in this immediate 

 work, like Fraunhofer, who was to follow him, to show a brilliant 

 promise, to do a brilliant work, then to be cut off before he could 

 enjoy its fruits. He had suggested a recondite method of working 

 out stellar paraHax from observations of the aberration and 

 velocity of light ; in practice it failed, from the extreme com- 

 plexity of the problem. 



He turned from methods to instruments ; the device he 

 imagined it seems independently to have occurred to a French 

 contemporary, Bouguer was not less ingenious and of far 

 greater value. It was simply the notion of having a sort of 

 binocular telescope with a single tube, showing two distinct 

 images ; it was so constructed that the images could be sepa- 

 rated or superimposed at the will of the observer. From this 

 it was possible to measure exceedingly minute angles. Perfected 

 in the hands of the famous instrument-maker Dollond, the 

 object-glass was simply cut in twain and mounted so that one 

 half might be made to slide past the other. As imagined by 

 Bouguer, the device was intended for the measure of the hori- 

 zontal diameter of the sun ; it thence derived its name of the 

 heliometer, or sun-measurer. 



So it remained, with but little wider application, until it 

 was taken up by a young German optician, Fraunhofer. He 

 had started out as an apprentice for a looking-glass maker, 

 deadly poor, with no parents and no one to help. He revealed 

 a genius for grinding and polishing mirrors and glasses ; eventu- 

 ally he rose to be the head of the optical department of a great 

 Munich firm. His name is more familiar as it is attached to 



