1877.] On the Chloride-of-Silver Battery. 519 



having become sufficiently enfeebled in comparison with the light of the 

 intercepted area of the gaseous envelope. The continuousness of the 

 gaseous envelope I contemplate is a physical, not a mere optical, con- 

 tinuity. 



Let A and B be two of the stars of such a cluster, C D a section of 

 the dispersed gaseous envelope enveloping the cluster. 



Then the light falling within the solid angle at O may be considered to 

 arise from the star A and the area C D of the gaseous envelope. 



Suppose the system removed to a greater distance. The light from 

 the star A is diminished in the proportion of the inverse square of the 

 distances, while that from C D is sensibly equal to that from CD'. The 

 question, therefore, whether such a cluster would, according to my views, 

 end in becoming a nebula or a mere optical cluster, would depend upon 

 whether it would be possible, with any supposed distribution of stellar 

 masses and vaporous envelopes, to diminish the brightness of the star A 

 below that given by the intercepted area C D of the enveloping surface, 

 before the star B has been brought to strengthen the beam of light which 

 gives the continuous spectrum. In the one case we should have absolute 

 irresolvability at that and all greater distances with any optical means at 

 our disposal ; in the other irresolvability, which might become resolv- 

 ability with increase of optical power. The case of two stars is of course 

 only taken for simplicity ; it is the proportional increase in other 

 cases which has to be considered. I have chiefly had in contemplation 

 nebulae like those of Orion and r) Argus, which extend over large angular 

 distances; but at great distances such nebulae might assume the character 

 of planetary nebulae. I see no difficulty in conceiving stellar clusters such 

 as those I contemplate, which would give rise to bright-line spectra ; and 

 I believe that the more the matter is examined the larger will be the 

 number of facts which will be found to group themselves around the 

 hypothesis which I have suggested. 



IV. u Experimental Researches on the Electric Discharge with 

 the Chloride-of-Silver Battery. — Part I." By Warren De 

 La Rue, M.A., D.C.L., F.R.S., and Htioo W. Muller, Ph.D., 

 F.R.S. Received August 23, 1877. 



(Abstract.) 



In the Journal of the Chemical Society, November 1868, we first pub- 

 lished an account of the " Chloride-of-Silver Battery." Since 1874 we 



2o 2 



