56o SCIENCE PROGRESS 



date ; again, if the spectra of the stars are photographed as 

 at the Harvard Observatory, then a new star might reveal 

 itself on inspection of a single plate by the peculiarity of its 

 spectrum. But neither of these methods was in the least 

 degree in our minds in the course of the Oxford work on the 

 map and the discovery was entirely accidental, as will be seen 

 from the following account. 



At the beginning of the year 1903 we were within sight of 

 the completion of the measures and hoped to reach it before 

 the end of the year. For several reasons the actual completion 

 was ultimately delayed beyond this date but that is a point 

 which does not concern us just now. In the hope of complet- 

 ing the measures before the end of 1903, we were making 

 great efforts to secure all the plates which had not yet been 

 taken. 



If the favourable season for taking a particular plate before 

 it " runs into daylight " is lost, we may have to wait nearly a 

 year before another opportunity recurs ; so that it was impor- 

 tant to obtain all the January plates in January 1903, not 

 leaving any gaps for January 1904. To expedite matters, when 

 there came a specially fine night or two, a large number of 

 plates were taken, which were set aside for development until 

 the good weather was gone. In England we have learnt to 

 prize these exceptional nights ; and it may be remarked in 

 passing that we occasionally get nights as good as anywhere 

 in the world, though the occasions are not so frequent as in 

 California, for instance. All too soon the indifferent weather 

 came, the plates were developed and, to our great disappoint- 

 ment, it was found that they were not satisfactory. There had 

 been an unfortunate failure in sensitiveness of the films, which 

 is apparently liable to happen in the manufacture of extremely 

 rapid plates ; when straining at the limit of sensitiveness some 

 very slight cause may produce a notable failure to reach that 

 limit. The disappointment was the greater because it was 

 practically the first of the kind ; throughout the whole work 

 the plates had been uniformly satisfactory, in spite of the 

 risks just mentioned, otherwise we might perhaps have been 

 on our guard against the shortcoming. There was, however, 

 nothing for it but to take the photographs again ; and if there 

 had been need for special exertion before, this need was now 

 much greater in consequence of the diminished time at disposal. 



