404 



Mr. W. J. Dibdin. 



[Apr 28, 



looked upon as something conveying energy along the conductor. 

 This part of the subject, although deduced from the present theory, is 

 shown to be true on Professor Poynting's own premisses. 



It is well here to call attention to what might prove confusing 

 otherwise. In what follows E, e, F, and some allied symbols stand 

 for certain external forces. But there are three different meanings 

 given in different parts of the paper to these symbols. They are 

 originally denned as the whole external forces of the different types. 

 But in treating of frictional forces, &c. (§§ 35 to 42), it is convenient 

 to regard them as meaning only those parts of the forces which are 

 due to friction and the like. Again, from § 50 onwards, it is conve- 

 nient to regard them as meaning only those parts of the forces which 

 are independent of friction and the like. This inconvenience is 

 incurred to avoid the greater evil of a large additional array of 

 symbols. 



With this exception,* and one or two other trifling ones, which are 

 noticed in their places, nowhere has the meaning of a symbol been 

 changed throughout the paper. 



IV. " Stellar Photometry." By W. J. Dibdin, F.I.C., F.C.S., &c. 

 Communicated by A. Vernon Harcourt, F.R.S. .Received 

 February 23, 1892. 



(Abstract.) 



Hitherto the determinations of stellar luminosity have been made 

 solely with regard to the relative intensity of the stars apart from 

 any reference to a known terrestrial unit. The various methods 

 which have been employed do not denote actual intensity, and the 

 present inquiry was, therefore, undertaken with the view of elucidat- 

 ing this question, especially with regard to those stars whose colour 

 has presented a difficulty. 



As a preliminary, the author prepared a standard series of artificial 

 stars of various colours and known intensity, in terms of the English 

 standard candle. These range in value from one candle to 0-000018 

 candle, and when placed at a distance from the telescope form a 

 standard series for comparison. 



The evaluation of the coloured lights was made by means of the 

 author's modified " star " disc, by the use of which comparisons of 

 various coloured lights can be readily obtained. 



* Since completing the paper I have discovered a notable exception, which is not 

 otherwise noticed than in this foot-note. It does not seem likely to lead to confu- 

 sion ; therefore I retain it. Most frequently in the present paper q stands for the 

 typical scalar coordinate of a dynamical system, but it is not infrequently used, as 

 in the former paper, for the quaternion of the rotation-operator q( )q~ 1 . 



