1866.] Mr. Huggins 07i the Spectra of Nehulce^ ^c. 



17 



M. Meyer's value is therefore nearly twice as great as that of this 

 paper, while that of Professor Stokes is only half as great. 



In M. Meyer's experiments, which were with one disk at a time in an 

 open space of air, the influence of the air near the edge of the disk is very 

 considerable; but M. Meyer (Crelle, 59; Pogg. cxiii. 76) seems to have 

 arrived at the conclusion that the additional effect of the air at the edge is 

 proportional to the thickness of the disk. If the additional force near the 

 edge is underestimated, the resulting value of the viscosity will be in 

 excess. 



7. Each of the forty experiments on dry air was calculated from the 

 concluded values of the viscosity of the air and of the wire, and the result 

 compared with the observed result. In this way the error of mean square of 

 each observation was determined, and from this the ** probable error " of 

 H was found to be '036 per cent, of its value. These experiments, it must 

 be remembered, were made with five different arrangements of the disks, 

 at pressures ranging from 0*5 inch to 30 inches, and at temperatures from 

 51° to 74° F. ; so that their agreement does not arise from a mere repetition 

 of the same conditions, but from an agreement between the properties of 

 air and the theory made use of in the calculations. 



February 15, 1866. 

 Lieut.-General SABINE^ President, in the Chair. 



The following communication was read : — 



" Further Observations on the Spectra of some of the Ncbulsej with 

 a Mode of determining the Brightness of these Bodies.^^ By 

 William Huggins, F.B.S. Received January 30; 1866. 



(Abstract.) 



In the first part of this paper the author continues his observations on 

 the spectra of nebulae and clusters. The results already presented by him 

 to the Royal Society are confirmed by his new observations, namely, that 

 •with his apparatus clusters and nebulse give either a continuous spec- 

 trum or a spectrum consisting of one, two, or three bright lines. The 

 positions in the spectrum of these lines are the same as those of the bright 

 lines of the nebulse described in his former papers. 



On account of the faintness of these objects the author w^as not able to 

 ascertain whether the continuous spectra which some of the nebulse give 

 are interrupted by dark lines in a manner similar to the spectra of the 

 sun and fixed stars. Some of these spectra appear irregularly bright in 

 some parts of the spectrum. 



The nebulse which follow have a spectrum of one, two, or three bright 

 lines ; in addition to which, in the case of some of them, a faint con- 



