306 Intelligence and Miscellaneous Articles. 



that we must have IB=KB 1 , B x being the projection of the point 

 B upon the horizontal KG- ; 2ndly, that there are as many fringes 

 of the rays Gr as of the rays B. from the curve BG to the vertical 

 IK. Consequently we have the evident relation 



42 (/o-/b)+™ X g='» X b> 

 whence 



m=6-81. 



Accordingly, besides the theoretic displacement, we must advance 

 6*81, which gives 12*06 fringes, and consequently, in conformity 

 with the experiment, 24 fringes for the displacement of the two 

 groups. 



With my biquartz of 60 millim. I have met with the same ex«- 

 aggeration. The thickness capable of producing the retardation of 

 one wave-length upon the middle ray of the spectrum is usually 

 estimated at 8 millim. ; the distance of the centres of the two 

 lateral systems ought therefore to be 15 fringes. Now it is found 

 that this distance reaches the figure 34 • and this exaggeration of 

 the displacement is explained as above. 



According to the formula we have 



m = 9'7, 



which number added to 7*5 gives 17'2, and consequently (very 

 approximately) 34 fringes for the total displacement. 



It results from the above that the abnormal displacement of the 

 two lateral groups receives a natural explanation, and that it cannot 

 be taken advantage of as an argument against Arago's hypothesis. 

 — Oomptes Rendus de VAcademie des /Sciences. September 12, 1881, 

 t. xciii. pp. 459-461. 



ON THE LIGHT OF COMETS. BY M. RESPIGHI. 



The appearance of the fine comets b and c 1881 has presented 

 to astronomers the opportunity of making new researches upon 

 the light of those stars ; and the results obtained by spectroscopy 

 and photography have appeared to confirm the idea that their light 

 is due in part to reflection of that of the sun, and in part to a 

 light which is the cometary matter's own. 



There is no doubt as to the first part of this conclusion, since to 

 the proofs already acquired Mr. Huggins has succeeded in adding 

 the photograph of Eraunhofer's lines in the spectrum of comet b 

 1881 ; but on the second point — that is to say, on the question of 

 knowing whether the comet emits a light of its own, due to a real 

 incandescence, I think there is no ground yet for giving an 

 affirmative decision. / Indeed it seems to me that the discontinuity 

 of its spectrum, as well as the bright lines or bands, may proceed 

 from the same cause, namely reflected light, if regard be had to 

 the modifications which that light must have undergone in passing 



