Intelligence and Miscellaneous Articles. 311 



twenty-eight feet of water : — 



Indicated horse-power. Probable speed, knots. 



7000 13 



8000 13-6 



9000 14-1 



lOOOO 14-6 



11000 1.5-1 



12000 15-6 



13000 16 



March 21, 1859. R. 



NOTE ON THE POLARIZATION OP THE LIGHT OF COMETS. 

 BY SIR DAVID BREWSTER. 



Although there can be no doubt as to the accuracy of the ob- 

 servations of M. Arago on the indications of polarization disco- 

 vered by him in the light of the comets from 1819 to 1835, there 

 is nevertheless nothing impossible in the supposition that the light 

 may have been polarized after arriving in the terrestrial atmosphere. 

 In fact, when we consider that light is polarized by refraction in 

 passing through the coats of the eye, that it is polarized by refraction 

 at the four or six surfaces of the object-glasses of an astronomical 

 telescope, and also in passing through the surfaces of its eyepiece, 

 and, lastly, that the light of celestial bodies undergoes a slight po- 

 larization by the refraction of the atmosphere, we are compelled to 

 admit that the problem of the existence of polarized light in the light 

 of comets is not solved. 



I am not aware that those who have observed traces of polarization 

 in the light of comets have noted the direction of the plane in which 

 it has been polarized ; nevertheless without some such observation 

 we cannot discover its cause. If the light be polarized in a plane 

 passing through the sun, the comet, and the eye, we must infer that 

 it is polarized by the reflexioti of the light coming from the sun ; if it 

 be polarized in an opposite plane, the polarization may be due to the 

 refraction of the atmosphere. If it be polarized quaquaversus, this 

 may be due to three causes ; namely, to refraction by the surfaces 

 of the object-glasses and eyepiece, to an imperfection in the anneal- 

 ing of the glass of which the lenses are formed, or to the fact of one 

 or more of the lenses being pinched in their cell. Supposing it to 

 be an effect of the first of these causes, the openings of the object- 

 glasses and eyepiece should be reduced to a central band, which 

 would eliminate the light polarized in an opposite plane, and leave 

 that which it polarized in a plane perpendicular to the direction. 

 By turning the telescope or the lenses, the direction of the polariza- 

 tion would be changed. 



If the polarization be produced by a defect in the annealing of the 

 glass of which the lenses are made, as appears to be the case in one 

 of Amici's telescopes mentioned by M. Govi, the existence of this 

 imperfection will be rendered evident by exposing the lenses to po- 

 larized light. 



