64 H. C. KUSSELL. 



The plate exposed for thirty minutes is however not so satis- 

 factory, for it should according to the rule, show with defined 

 discs stars of the 14th magnitude of Argelander's scale. In 

 Herschel's monograph on this cluster he has eleven stars of 14th 

 and four of 15th magnitude, of Argelander's scale, of these eight 

 are invisible, six are visible but not measurable, and only one is 

 " measurable," and some stars of even the 12th and 13th magni- 

 tudes that are not measurable. The plates were exposed one after 

 the other, on a night that seemed to be uniform, and when the 

 two minutes plate was a success the thirty minutes one ought also 

 to have been. I give the result of this experiment to shew one 

 serious difficulty that besets the work; at first sight the foregoing 

 results look like a failure of the method, but I find that these 

 faint stars in Herschel's list are either much fainter than he took 

 them for, or they are coloured stars. This was proved by taking 

 a photograph of the same object and giving three hours exposure. 

 Even then most of the stars referred to above are far too faint to 

 measure, although they can be seen plainly enough in nearly 

 every instance, and the photograph, hurriedly examined to see if 

 the faint stars were on it, is found to contain at least twenty 

 more faint stars which Herschel did not see. This example will 

 serve to show you better than any statement, the difficulty to be 

 met in following the adopted rule, viz., if two minutes exposure 

 records stars of the 11th magnitude, then thirty to thirty-five 

 minutes should record those of 14th, for here in the case of a well 

 known cluster, with every star recorded by careful observers^ it 

 is found that the rule fails, and the question arises, did Herschel 

 over-estimate the magnitude of these stars, or are they coloured. 

 Over nearly the whole surface of the sky we have no record of 

 stars below the 9th magnitude, and therefore no means of finding 

 whether the photographs will really record what is desired, that 

 is stars of 14th magnitude. It is obvious therefore, that more 

 experiments will have to be made upon well known clusters to 

 determine the time necessary for the purpose of making certain of 

 14th magnitude stars. When that is done however, we shall have 



