ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 



145 



An exaggerated protuberance of the central portion of the moon might 

 result from the combination of two pictures obtained, at two epochs of 

 maxima, in directions diagonally opposite, and the moon would appear some- 

 what egg-shaped. We may convince ourselves that this would be the case, 

 by viewing, in the stereoscope, two suitably drawn orthographic projections 

 of the lines of longitude and latitude of the sphere, especially if we purposely 

 exaggerate the angle still more ; for example, if we make the libration in 

 latitude the double of what it is in reality. 



At the meeting at Leeds last year, there were exhibited some of my stereo- 

 scopic lunar pictures 8 inches in diameter, and an apparatus constructed 

 expressly for viewing them. The instrument is of similar construction to 

 Wheatstone's reflecting stereoscope ; but, the objects being transparent, the 

 usual arrangements and adjustments are considerably modified. Prisms with 

 slight curvatures worked on their surfaces are employed, instead of mirrors, 

 for combining the pictures which can be revolved and moved horizontally 

 and vertically in order to place them in the true position. The effect of 

 rotundity is perfect over the whole surface ; and parts which appear like 

 plane surfaces in a single photograph, in the stereoscope, present the most re- 

 markable undulations and irregularities. 



Light and Shade in the Photograph as compared with that of the Optical 

 Image. — Portions of the moon, equally bright optically, are by no means 

 equally bright chemically ; hence the light and shade in the photograph do 

 not correspond in all cases with the light and shade in the optical picture. 

 Photography thus frequently renders details visible which escape observa- 

 tion optically, and it therefore holds out a promise of a fertile future in sele- 

 nological researches ; for instance, strata of different composition evidently 

 reflect the chemical rays to a greater or less extent according to their nature, 

 and may be thus distinguished f. The lunar surface very near the dark limb 

 ia copied photographically with great difficulty, and it sometimes requires an 

 exposure five or six times as long, to bring out completely those portions 

 illuminated by a very oblique ray, as others, apparently not brighter, but 

 more favourably illuminated : — the high ground in the Southern hemisphere 

 of the moon is more easily copied than the low ground, usually called seas, 

 which abound in the Northern hemisphere: from these circumstances I 

 ventured, in another placej, to suggest that the moon may have an atmo- 



* These diagrams should be 2| inches from centre to centre to give a stereoscopic 

 picture. 



t Professor Phillips has also noticed this difference between the visual and actinic 

 brightness of portions of the lunar surface. Report of the Brit. Ass., 1853, Section A. p. 16. 



J Monthly Notices Roy. Ast. Soc. vol. xviii. pp. 18 and 111. 

 1859. L 



