58 



The Moon. 



such as are now becoming both cheap and common, will bring out 

 details enough to occupy, in their close study and careful deli- 

 neation, the leisure hours of many a long year. This branch 

 of astronomy will be found peculiarly within the reach of the 

 numerous class of amateurs whose telescopes are furnished 

 neither with micrometers nor clockwork motions • and it is 

 one in which they may do good service, provided only the 

 judgment of the eye is good in estimating proportions, and the 

 hand fairly practised in the most desirable acquirement of 

 drawing. This latter is, indeed, an acquirement — accomplish- 

 ment, in the ordinary sense, seems too trivial a name for it — 

 of more value than the inexperienced may be aware of; for it 

 is found that the habit of representing what is seen reacts 

 upon the mode of seeing, and the accuracy of the hand in- 

 creases the discrimination of the eye, so that a practised 

 draughtsman distinguishes more, especially in a complex object, 

 than one ignorant of design, even with a naturally keener 

 sight. The micrometer is by no means so necessary in this 

 pursuit as in the observation of double stars, since, however 

 desirable it may be to fix the principal points in a lunar survey 

 by actual measurement, the details may be quite as well filled in 

 by hand ; while, should a micrometer be employed, the trouble 

 of arranging an artificial illumination is avoided. Schroter 

 employed, in his numerous delineations, a contrivance called 

 a "projection machine," which consisted merely of a white 

 surface, divided by parallel lines into numerous small squares, 

 placed at a convenient distance from the eye, and exposed to 

 a suitable illumination. His telescope being of the Newtonian 

 construction, this surface was supported by a bar fixed perpen- 

 dicularly to the tube at its mouth, and thus he was enabled 

 to view at the same time any lunar region with the right 

 eye in the eye-piece, and the divided surface with the left 

 eye, unaided, across the open end of the telescope, so that, 

 by means of a paper similarly divided into squares, he could 

 make drawings with greater accuracy than could be attained by 

 the eye alone, though inferior to that resulting from the use of 

 the micrometer. This method of projecting the image on a 

 divided scale (whence the name of the apparatus) is now found 

 useful for microscopic purposes : it would be difficult to attach 

 it to an achromatic telescope ; and if measurement is desired, 

 the object might be more conveniently attained by inserting in 

 the focus of the eye-piece a little divided glass scale, which may 

 be obtained of Messrs. Home and Thornthwaite, 121, Newgate 

 Street ; or the photographic image of a scale, which I had tried 

 by a friend some time ago with a very fair prospect of success, 

 and which has since, I understand, been advertised for sale. 

 However, a practised eye, capable of estimating pretty sharply 



