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the Milky Way is in any way related to these possibilities. 

 Hence it will be understood why counting all the stars on 

 a plate, trivial as it might appear at first sight, has been an 

 important operation in connection with the Star Map. It is 

 usually done with a " billiard-marker." Those who play 

 billiards have various devices for marking the score, one of 

 which is a small apparatus, held in the hand, provided with two 

 or four little springs. On pressing one of these springs one 

 of the numbers shown on the face of the apparatus changes and 

 by a series of clicks the score is registered as required. Now 

 one of the astronomers who was taking a share in the chart 

 had "misspent his youth" and accordingly knew of this 

 apparatus and saw that it would be useful for counting star 

 photographs : for the plate can be passed in review under 

 a microscope with one hand, while with the other hand a 

 click can be made, without removing the eye from the 

 microscope, whenever a star is seen. The success of the 

 performance has been sufficient to cause " billiard-markers " 

 to be exported to distant astronomical observatories, where 

 there is, as a matter of fact, no billiard-table. 



But how comes it that different exposures are given in the 

 actual course of the work on the Star Map ? Is not uniformity 

 one of the essential features of the scheme ? It was certainly 

 the original idea that a particular length of exposure should 

 be selected and adhered to throughout : fifteen minutes found 

 most favour. It was considered, in the light of the experience 

 available, that fifteen minutes would give stars as faint as the 

 14th magnitude, and was not so inconveniently long as to be 

 a tax on the observer. After this had been practically settled 

 it was remarked that the bright stars would, with so long an 

 exposure, form very large images, the centre of which could 

 not be accurately determined. Now it is important to measure 

 these bright stars accurately, for we already know their 

 positions in the sky with some precision, and they are therefore 

 useful reference marks for the others. Hence it was decided 

 to take another series of plates with a shorter exposure 

 (ultimately fixed at six minutes), on which the images would 

 not be so large. This bifurcation of the enterprise was 

 subsequently developed in both prongs. To the shorter 

 exposure, two others shorter still, of 3 minutes and 20 seconds, 

 were afterwards added, for good reasons which we need not 



