aio 



(3.) That all interposed media (including those impermeable to 

 liglit), so far as tried, raise the index of refraction. (4.) That all 

 the refrangibilities are inferior to that of the mean luminous rays. 

 (5.) That the limits of Dispersion are open to farther inquiry, but 

 the dispersion in the case of the sources of low temperature ap- 

 pears to be smaller than in that from luminous sources. 



2. Remarks on the Illustrations presented by the Surface of 

 the Moon, respecting certain Geological Phenomena. 

 In a Letter to Sir J. Robison, K. H, &c. from Profes- 

 sor Nichol, University of Glasgow. 



Professor Nichol conceives that, as the upheaving cause is a ge- 

 neral or cosmical phenomenon, no true or complete theory of it can 

 be deduced from a review of its merely terrestrial actions ; and in 

 the notice read to the Society, he called the attention of geologists 

 to a few general but important inferences, from the peculiar form 

 of the elevations in the moon. 



The author introduced his observations, by adverting to the fol- 

 lowing facts, as now ascertained and generally known concerning 

 these elevations. 



1. There are ranges of mountains in the moon, but they are com- 

 pai-atively few. Some of them, especially the ^Apennines, with 

 their attached extensive highlands, bear a close resemblance to many 

 terrestrial ranges. 



2. The chief form of elevation is the circular. The moon's sur- 

 face is absolutely studded with crater-forms, — objects recently exa- 

 mined with much care, and represented in an admirable map, by 

 Beer and Miidler of Berlin. Their diameters vary from fifty or 

 sixty miles to the minimum visibile, and they greatly increase in 

 number as their magnitude diminishes. They are all marked by 

 this singular feature — their interior is deepei' than the general lunar 

 surface, as if the elevation had been occasioned by a mass of the 

 moon being blown out from a single point of disturbance. It is to 

 be noticed, likewise, that the external effect of this disturbance in 

 no case extends very far. In some instances the crater-ridge rises 

 at once out of the flat body of the moon, without appearing to aflfect 

 it almost at all; but even where this surface has been raised and dis- 

 turbed, as in the case of the crater Copernicus, the sphere of the 

 action seems exceedingly confined, when compared with the trans- 

 verse extent through which a disturbance is felt on the earth. 

 There is nothing on the earth to which these formations can be 



