The Moon. 197 



eye sensitive to light, much comfort may be experienced from 

 the adoption of a screen-glass, such as is used for solar observa- 

 tions, but, of course, of a much lighter hue : a pale green has 

 been recommended by Challis as very suitable. High mag- 

 nifiers are seldom of much value ; they contract the field 

 unpleasantly, and increase the apparent motion of the object, 

 unless a driving-power is at hand. From 100 to 300 times 

 may be mentioned as most generally serviceable. Beer and 

 Madler never exceeded the latter in their original investigation 

 with an aperture of nearly 4J inches, though Madler, after 

 succeeding W. Struve at Dorpat, employed powers of 600 and 

 even 1100*, with the great achromatic of 9'6 inches, in that 

 observatory. 



We begin with one of the most familiarly-known spots, 

 the — 



MAEE CEISIUM. 



The remarkably well-defined grey plain, marked A in our 

 Index -map, has received from Riccioli the name of Mare 

 Crisium,t " the Sea of Crises," by which, as there is nothing 

 of an astrologico-political character in his nomenclature, he 

 probably meant changes of weather ; and, if so, has, as far as 

 he could, commended it to the especial attention of English 

 astronomers. It is so distinctly and strongly bounded as to be 

 always easy of identification, lying near the W. limb, and not 

 far N. of the lunar equator : and we cannot wonder at the im- 

 pression of the earlier observers, who imagined, in such a 

 striking level, the exact counterpart of a terrestrial sea, or 

 great lake encircled by a rampart of mountains. Such, how- 

 ever, was not the full conviction of Hevel, though he adopted 

 the name as the closest approximation that he could find ; and 

 such an idea will not be revived in the present day, so multiplied 

 and so distinct are the roughenings of the surface which modern 

 instruments will show, and so clear the view into the depths of 

 craters, which would be the chief receptacles of any fluid exist- 

 ing there. J The " Mare " before us is evidently a very inte- 

 resting one; its oblique position, however, subjects it to so 

 great a foreshortening that its interior is studied to much less 

 advantage than if it occupied a more central situation. From 

 its nearness to the edge of the disc its apparent form is much 



* These were, however, probably much exaggerated. Encke found that the 

 GOO power on a similar achromatic at Berlin, by the samo maker, proved to be 

 only 400 by the dynameter. 



t Certainly little imagining that any future astronomer would ever quote its 

 genitive case as " Maro Crisii," which, however, B. and M. have done in one 

 instance, p. 194. 



X Arago, however, thinks this inference not conclusive, as the uneven, craggy 

 bottom of our oceans may be distinctly seen from a great height. It might be 

 added that fresh water would be still more translucent. 



