374 SCIENTIFIC KECOED FOR 1885. 



aud freedom from liability of derangement. In addition to these qual- 

 ities, Professor Pritchard has been careful, in bis use of it, to free it 

 from systematic errors arising from the continual use of the same part 

 of the wedge for the same star. iSTot only has the coefficient of absorp- 

 tion been separately determined for every point of the wedge, but the 

 system of observation adopted has involved the employment of four dif- 

 ferent apertures of telescope in conjunction with two wedges, and by 

 two different observers. Each determination of magnitude, therefore, 

 is the mean of twenty observations, a set of five observations being taken 

 with each of what are practically four different instruments. When to 

 this is added the circumstance that the determinations of magnitude are 

 differential only, and that therefore three complete sets of extinctions 

 were made of the standard star Polaris every evening, and that for the 

 better determination of the atmospheric absorption and of the magni- 

 tudes of southern stars a considerable portion of the observations were 

 made at Cairo, it becomes evident that the work of determining the 

 magnitudes of nearly 3,000 stars which Professor Pritchard, with liis 

 assistants, Messrs. Plummer and Jenkins, has here accomplished, is one 

 of very considerable dimensions indeed. 



"The convenience of Professor Pritchard's photometer, and the magni 

 tude of the work he has undertaken with it, stand beyond dispute ; but 

 the delicacy of the wedge is another question. And here we are met 

 with the circumstance that the observations seem to show little or no 

 evidence of any effect due to changes in the sensitiveness of the ob- 

 server's eye, to personality, to moonlight, and only to a small extent to 

 color in the star observed ; and this unexpected and remarkable result 

 has called forth not a little criticism, for as the construction of this 

 photometer is not such as to lead us to expect that it would be wholly 

 free from errors of these kinds, a doubt seems to be thrown upon its 

 sensitiveness. Professor Pritchard has replied to these criticisms in 

 the frankest manner, showing that they had not escaped his notice ; 

 but whether he has quite refuted them is a point which we may, how- 

 ever, well think still sub judice. Probably the wise arrangement by 

 which an ordinary evening's work is confined to three hours or less 

 will largely explain the absence of deviations due to the first-named 

 cause, and with regard to the third it is most likely that observations on 

 bright moonlight nights or of stars near the moon have been gener- 

 ally avoided. The observations of Polaris which Professor Pritchard 

 has brought forward in this connection scarcely touch the real diffi- 

 culty. The substantial accuracy of the Oxford star magnitudes is, how- 

 ever, shown by the close agreement which they bear to determinations 

 made at other observatories and by widely different methods, and it is 

 possible that the future may show that the exceedingly small devia- 

 tions of the individual observations are a true index of the minuteness 

 of their errors." (E. W. Maunder, Observatory.) 



