THE GREAT STAR MAP 253 



stop to notice: and the longer exposure of 15 minutes was 

 extended to 20 minutes, then to 30 minutes, then to 40 minutes, 

 and ultimately to one hour, subdivided into three separate 

 exposures each of 20 minutes. We need not follow the reasons 

 in detail, it is sufficient to remark that for the originally 

 projected uniform exposure of 15 minutes a series has been 

 substituted, for reasons more or less good, of four or five 

 exposures ranging from 20 seconds to 40 minutes. The ratio 

 of 120 to I between the longest and shortest corresponds 

 roughly to 7 stellar magnitudes and hence we have material 

 for studying the variation of total stars per plate over a con- 

 siderable range. 



The particular proposal to giv^e three separate exposures 

 of 20 minutes each has had an unexpected result of an interesting 

 kind. The three exposures are arranged in the form of a little 

 triangle, so that each star is represented by three little dots in 

 this figure. Now these long exposure plates are not to be 

 measured but are intended for reproduction as charts. They 

 represent the most expensive part of the project, each of the 

 eighteen shares costing some ^10,000 in all. At Oxford and at 

 some of the other less wealthy observatories it has not been 

 possible to undertake this expensive part of the work, but the 

 French are particularly interested in it, as a consequence of 

 earlier national projects of the same kind ; the French Govern- 

 ment has most generously undertaken the reproduction of the 

 " Charts " in the Oxford zone and in some others. These charts 

 are being very beautifully and carefully reproduced in Paris by 

 heliogravure. As each chart is printed it is patiently examined 

 for possible defects, to see for instance whether any of the 

 images on the original plate are missing or whether any 

 accidental blots could be mistaken for stars. One test applied 

 by M. Jules Baillaud, who has special charge of this work, is 

 a comparison of the three separate images of each star to see 

 that they resemble each other; if they do not there is usually 

 some defect in the reproduction calling for correction. But what 

 was his surprise one day to find a distinct difference in the 

 images of a particular star which was not a fault of reproduction 

 but was apparent in the original plate itself! The conclusion 

 was forced upon him that the star had varied in brightness 

 between the exposures. The interval is usually so short that 

 such an occurrence would be startling, but in this particular case 



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