554 SCIENCE PROGRESS 



possible on opposite sides of the earth. A few sentences ago 

 we compared a pair of eyes to a pair of telescopes pointed at 

 the same object from opposite sides of the earth ; but a single 

 telescope may be made to serve the purpose of a pair, since 

 the rotation of the earth carries it round during the night 

 from one side to the other ; and this will explain the sunset 

 and sunrise exposures to the planet Eros. In this way many 

 hundreds of such photographs were obtained. 



The next thing to do was to measure all these photographs 

 accurately, in order to determine the place of the tiny planet 

 among the stars. This place was of course changing con- 

 tinually, owing to the movement of the planet round the sun 

 and indeed owing to a similar movement of the earth also. 

 But it was possible to devise methods of allowing for this 

 movement and correcting the measures for it. There would 

 remain the displacement due to " parallax," that is to say, to 

 the finite distance of the planet which it was required to 

 measure. The greatest amount of such displacement was about 

 23'' — about one-hundredth part of the moon's apparent diameter ; 

 which it was desired to measure if possible to the thousandth 

 part of itself. Hence the measurement required a new order 

 of accuracy ; the apparatus already in use for the Great Star 

 Map needed modification in essential details for this new 

 enterprise. Moreover it is a familiar fact in scientific work 

 that, when we proceed to the next decimal place, we always 

 encounter a number of unforeseen difficulties of all kinds ; and 

 the measurement of the Eros plates was no exception to this rule. 

 Some of these difficulties arose in the course of the measurement 

 at the separate observatories and were vanquished as they 

 arose. Particularly was this the case at our national observatory 

 at Greenwich, where a complete determination of the distance 

 of the planet was made from the Greenwich plates alone, 

 without help from those of any other observatory and with very 

 satisfactory results. But to get the full advantage from all the 

 many photographs taken it was necessary to co-ordinate all 

 the measures made at the different observatories, which brought 

 to light a new crop of difficulties. It is to the lasting credit 

 of Mr. A. R. Hinks, of the Cambridge University Observatory, 

 that he undertook, as a volunteer but with the full approval 

 of the President of the Committee charged with the work, 

 to collect and co-ordinate all the results. The labour was very 



