196 



The Moon. 



different lengths of shadow ; and should any observer, pos- 

 sessing a good micrometer, and facility in the use of it, wish to 

 offer such a. contribution to selenography, he will find all the 

 necessary formulas in Der Monti (The Moon) of Beer and 

 Madler. 



As to the instrument to be employed, many of the lunar 

 features are so conspicuous, that any good telescope will suffice 



(ALHAZEN 



'! lit- ;i hove; diagram of ihcMare Crisium contains the principal features in the 

 map of ]3. and AI. ; the portion, however, surrounded by a doited lino is 

 altered to correspond wit) 1 my own observations.] 



bo sln.w them. Of course, a Larger instrument would bo pre- 

 ferable; but even b 2 -inch object-glass with a good astro- 

 nomical eye-piece will reveal wonders enough to be a constant 

 source of interest. In the case of a very large aperture and an 



