46 Mr. II. A. Sampson. [Dec. 11,. 



Professor E. C. Pickering informs me " that photographs have been 

 obtained at the Harvard College Observatory of all the stars hitherto 

 discovered whose spectra consist mainly of bright lines and are of the 

 class discovered by Rayet. Part of these have been photographed at 

 Cambridge, and the remainder in Peru." He states that they may be 

 divided into three sub-classes, according to the characters of their 

 fifteen bright lines. He says, further: "Photographs of the spec- 

 trum of planetary nebulae have also been obtained. They resemble 

 closely the spectra described above, except that the line 500 is 

 strongly marked ; 470 is seen in most of them, while the lines due to 

 hydrogen are also bright." 



It would seem that Professor Pickering's photographs do not permit 

 him to distinguish the different positions of the bright blue band in 

 some of these stars, for he gives for all the stars the same position, 

 namely, X 470. 



We regret that the insufficiency of our instrumental means has left 

 our examination of the spectra of these stars less complete than we 

 could wish. Our observations appear to us, however, to be conclusive 

 on the main object of our enquiry, namely, that the bright blue band 

 in the three Wolf-Rayet stars in Cygnus, and in D.M. +37 3821 y 

 is not coincident with the blue band of the Bunsen flame. 



V. "On Stokes's Current Function." By R. A. SAMPSON, 

 Fellow of St. John's College, Cambridge. Communicated 

 by Professor GREENHILL, F.R.S. Received November 24,. 

 1890. 



(Abstract.) 



In Maxwell's 'Electricity and Magnetism,'* a view is put for- 

 ward, in accordance with which we may regard any irrotational 

 motion in a perfect liquid, for which the velocity potential is a solid 

 zonal harmonic, as due to the juxtaposition at the origin, and upon 

 the axis of symmetry, of sinks and sources. 



But, in a liquid, any irrotational motion which is symmetrical 

 with respect to an axis gives a velocity potential which may be 

 expressed as a sum of a series of solid zonal harmonics, their common 

 axis being the axis of symmetry, and their origin arbitrary, provided 



multiple image of four or five stars is surrounded by five bright and seven faint 

 stars ; all within a radial distance of 82" of arc measured from centre to centre 

 of the multiple star. The multiple image measures 55" in length and 19" 

 in breadth. 



" No. 3956. Its photo-image is 27" in diameter. It is encircled by three 

 stars of lesser brightness, and six faint ones within a radial distance of 59", i.e., 

 there are nine stars within a radial distance of 59"." DPC. 5.] 



* Vol. 1, chapter ii. 



