THE GREAT STAR MAP 555 



heavy and has occupied a large part of his working time 

 during ten years. The difficulties which cropped up were new at 

 every turn and great ingenuity was called for in overcoming them. 



One such difficulty may be mentioned in illustration. It 

 has been remarked above that, when stars are observed 

 visually, there is apt to be a " magnitude equation," i.e a 

 difference between the records for bright and faint stars but 

 that the introduction of the photograph seemed to offer a check 

 on these errors, being itself free from them. On this assumption 

 Mr. Hinks proceeded to treat the measures of the Eros plates ; 

 but on comparing the measures at different observatories, he 

 found between them just such differences as affect old visual 

 observations under the head of magnitude equation. The 

 differences were not so large perhaps but were nevertheless 

 sensible. It may occur to the reader to inquire whether the 

 differences arose in the measurements of the plates, which 

 were of course made visually ; but such a possibility was 

 eliminated by the method of measuring each plate twice over, 

 as has been explained in a previous article, the plate being 

 turned completely round for the second set of measures ; any 

 magnitude equation would affect the two sets of measures in 

 reverse directions and could thus be both detected and 

 eliminated. This alternative being ruled out, it followed that 

 the error must be in the plates themselves and it was a very 

 disturbing discovery to find that we had not, as had been 

 hoped, freed ourselves from such a kind of error by the 

 introduction of photography. 



Some comfort was forthcoming from the further discovery 

 that many of the plates were sensibly free from this error, but 

 these only increased the puzzle. What could be influencing 

 those which showed unmistakable traces of it? Ultimately 

 the cause was found in a faulty lens or rather in the faulty 

 arrangement of the pair of lenses which go to make up the 

 object-glass of a telescope. We have now realised that this 

 arrangement must be carefully made and that faults render us 

 liable to this old trouble ; but it also seems probable that with 

 care on the part of the instrument maker, the trouble can be 

 avoided or, at any rate, rendered conveniently small. It is 

 easy to sum up in a few words in this way the net result of 

 the investigation ; but the investigation itself was a long and 

 tedious one and is perhaps even yet scarcely complete. It 



