Intelligence and Miscellaneous Articles. 179 



more than a single cometary spectrum, slightly modified in different 

 comets by differences of pressure and temperature. 



It would now appear from my observations that Brorsen's comet 

 also must fall into line with the rest. 



I am entirely at a loss how to explain Mr. Huggins's result. It 

 can hardly be that the comet has really changed its spectrum in 

 the meanwhile ; and a careful reading of his account (Proc. Roy. 

 Soc. vol. xvi. p. 388) gives no light as to how an error could have 

 crept into his work ; on the other hand, every precaution would 

 seem to have been taken. However this may be, I am quite positive 

 as to the accuracy of my present result — that the middle band of 

 the spectrum of this comet now coincides sensibly (to a 'one -prism 

 spectroscope) with the green band in the hydrocarbon spectrum. 



The comet is moving very nearly in the path assigned by the 

 ephemeris of Schulze. It is easily visible in the 3-inch finder of 

 the equatorial, and in the telescope itself appears as a round nebu- 

 losity, between 30" and 40" in diameter, without definite nucleus, 

 though much brighter in the centre. Before the new moon a faint 

 tail was visible, about one half degree in length. It appeared like 

 a thin streamer, much narrower than the head of the comet, perfectly 

 straight, and directed from the sun. — Silliman's American Journal, 

 May 1879. 



ON STOKES'S LAW. BY M. LAMANSKY. 



It is known that Stokes^ in his important researches on fluo- 

 rescence, laid it down as a principle that the refrangibility of the 

 light emitted by fluorescence is less than that of the exciting rays. 



Stokes's law has lately been called in question by Lommel, who 

 in several memoirs published in the Annalen der Physih has sought 

 to show that it is not a general law, and that there are cases in 

 which the fluorescent light possesses a greater refrangibility than 

 that of the incident light which excites the fluorescence. The 

 results obtained by Lommel in his experiments have been confirmed 

 by B. Brunner (of Prague) and Lubarsch (Berlin) ; but Hagenbach, 

 the author of some very accurate studies on fluorescence, was not 

 able to arrive at the same results as Lommel. 



After repeating the experiments described in the memoirs of 

 these physicists, it appeared to me necessary, in order to decide 

 this controverted question and give an experimental proof of 

 Stokes's law, to discover a method that would permit direct measure- 

 ment of the refrangibility of fluorescent light and its comparison 

 with that of the incident light exciting the fluorescence. For this 

 purpose the light of the exciting rays operated on must be perfectly 

 homogeneous. This can be attained by making use of the method 

 first employed by Maxwell and Helmholtz, and afterwards by several 

 other physicists. The following is the method I have used in 

 these researches : — 



The solar rays reflected by a heliostat were concentrated by an 

 achromatic lens upon a slit; behind which were placed two flint-glass 



