02 The Surface of the Moon. 



difficulty in identifying Herschel 17s nebula in this region. He 

 questions also the variation of another nebula mentioned by 

 Schmidt. Three, however, of Sir W. Herschers nebulas — two 

 of them of his first or brighter class, the other of his second — 

 he has failed to find. The proper motion, which has been sus- 

 pected by some astronomers in nebulas, he thinks improbable. 

 Prom time to time we hope to be favoured with further accounts 

 of his progress in his most difficult, but most valuable and in- 

 teresting, undertaking. 



No objects (if we except comets, to which they often bear 

 a curious resemblance) are more benefited than nebulas by an 

 increase of optical light. This is a natural result of the faint- 

 ness and haziness which is their usual characteristic ; and for 

 their mere discovery or recognition, defining power is of less 

 importance than breadth of aperture. When, however, the 

 attempt is made to detect their starry composition, accurate 

 definition resumes its usual supremacy ; and increase of power 

 is here important, as tending, by separating the minute points 

 of light, to draw them out of the general haze. At the v same 

 time the want of contrast with the open sky attendant upon 

 the contracted field of a high power is a serious disadvantage, 

 and deep magnifiers should only be employed after the eye has 

 become familiar with the object in the use of low and progres- 

 sively increasing powers. The brighter nebulas may be seen 

 with so small an instrument that it seems singular that so few 

 of them were noticed before the time of Messier. They are 

 often perceptible enough in the fields of our finders. With an 

 object glass of only lj inch, in youthful days, I mado out, un- 

 doubtingly, though of course dimly, several of these objects, 

 and could have seen more had I known where to look for them. 

 An object glass of 3f inches showed a great number very 

 satisfactorily ; and with 5| inches the brighter clusters aro 

 beautifully developed. They require, of course, from the mi- 

 nuteness of their components, a darker sky than the majority 

 Of tin' double stars included in our list, and in haze, twilight, 

 or moonlight, the search would be to little purpose. A popular 

 catalogue of the more remarkable of these interesting objects 

 ^•, ill shortly follow. 



XHH SUEFACE OF THE MOON. 

 In reverting to this subject, which has now, from the pres- 

 sure of other interesting matter, been intermitted for a con- 

 siderable time, wo would [joint out to the inexperienced 

 observer that, although, in mastering the difficulties which 

 have been already indicated as existing in the accurate study 

 and delineation of the BUrfaoe of the moon, ho will have ad- 

 vanced very Far towards the attainment of his object, there 

 i ill remains a source of perplexity, which will not indeed cau so 



