PORTION OF THE COSMOS. THE SOLAR DOMAIN. 353 



ing to the many and various observations made by Arago with 

 his polariscope, the light of the Moon contains polarised light, 

 which is most distinctly traceable during the first quarter of the 

 Moon and in the grey spots on its surface : for example, in the 

 large, dark, sometimes somewhat greenish, wall-surrounded 

 plain called the Mare Crisium. Such plains are mostly tra- 

 versed by long low ridges of hills, offering those angles 

 of inclination of the surface which are required for the 

 polarisation of the reflected solar light. The dark tone 

 of colour of the adjacent parts seems, moreover, to render 

 the phenomenon still more sensible by contrast. As regards 

 the shining central mountain of the group called Aristarchus, 

 on which observers have repeatedly imagined that they saw 

 volcanic action taking place, it showed no stronger polarisa- 

 tion of light than did other parts of the Moon's surface. 

 In the full moon no mixture of polarised light was per- 

 ceived; but during a total lunar eclipse (31st May, 1848) 

 Arago remarked undoubted signs of polarisation in the 

 reddened disk of the Moon, a phenomenon of which we 

 shall have occasion to speak presently (Comptes rendus, 

 T. xviii. p. 1119). 



That the light of the Moon does produce heat is (like so 

 many others due to my celebrated friend Melloni) among 

 the most important and most surprising discoveries of our 

 century. After many unsuccessful attempts, from La Hire 

 to those of the acute Forbes ( 564 ), Melloni, by means of a 

 lens (lentille a echelons) of three feet diameter, made for the 

 Meteorological Institution on the Cone of Mount Vesuvius, 

 succeeded, in different phases of the Moon, in observing 

 must satisfactory indications of an elevation of temperature. 

 Mosotti, Lavagna, and Belli, Professors of the Universities 



