232 



Prof. J. Phillips on some Parts of 



[Jan. 16, 



ing- paper, a well-arranged light, and no necessity of changing position. I 

 made in this manner the drawing of Gassendi which is marked No. 2. 



My next attempt was made in the same situation with a fine tele- 

 scope by Cooke, of 65-inch aperture and 11 feet sidereal focus, mounted 

 equatorially, in the old English mode, and carried by clockwork. With 

 this excellent arrangement I was enabled to use photography very success- 

 fully, and to obtain selenographs 2 inches across in 5® of time. The draw- 

 ing of Gassendi, No. 3, was made with this instrument (1853). 



From these experiments the conclusion was obvious : — that for obtaining 

 good drawings of the moon, convenient mounting was actually more im- 

 portant than great optical power ; and that for such a purpose it was desi- 

 rable to increase in every way the comfort of the observer, and furnish him 

 with special arrangements for his own position and the placing of his draw- 

 ing-board and light. 



Having been called to reside in Oxford (1853-54), my plan for con- 

 tinual work on the moon was entirely cut through ; it was impossible to 

 mount a large instrument near my dwelling till (in 1860) the ground was 

 arranged about the museum, so as to give me the requisite space and secu- 

 rity close to the house which had been appointed for me by the Univer- 

 sity. I then arranged with Mr. Cooke for a new telescope of 6 inches 

 aperture, to be protected in a well-planned observatory, the construction of 

 which was aided by the E.oyal Society. I now propose to give a short 

 notice of some of the results of my work with this instrument, in connexion 

 with remarks on the most advisable course to be followed by other surveyors 

 of the moon. 



In making drawings of ring-mountains on different parts of the moon's 

 disk, the artist will be much aided by a projection of the mountain-border 

 on the scale intended, from a few measures, with its proper figure due to 

 the latitude and longitude. Eye-drafts not thus controlled are apt to be- 

 come absurd, by the heedless substitution of an ideal circle for a real 

 ellipse. Thus I have seen Gassendi forgetfully represented by more than 

 one skilful artist. Even with the advantage of such a projection (of which 

 I give an example for Gassendi, No, 5) very considerable difficulties occur. 

 One is the variation of outline caused by the displacement of the boundary 

 of light and shade — first when the incidence of light varies through different 

 angles of elevation of the sun, and next when the moon's position causes 

 her to receive the light at the point observed on a different lunar azimuth. 

 Even on so great a ring as Copernicus the variation of the outline as given 

 by different artists is remarkable — hardly any one agreeing with what is 

 really the most accurate drawing of all, that by P. Secchi ; and that repre- 

 sents, not a simple ring, but a seven-angled outline. Dates must always 

 be annexed to the figures ; and as it is rarely possible to complete a good 

 drawing of a large crater, except in two or three lunations or more, it 

 becomes very essential that a bold free sketch be made of the moon's 

 shadov^^s to control the special work. (No. 6 is given as an example.) 



