ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 



143 



would no longer be semi-circles, but semi-ellipses, whose conjugate diameter 

 is equal to 0-9965, and whose transverse diameter is inclined 90° to the pole. 



Stereoscopic Pictures of the Moon. — Taking advantage of the libration, we 

 may, by combining two views taken at sufficiently distant periods, produce 

 stereoscopic pictures which present to the eyes the moon as a sphere. It 

 has been remarked by the Astronomer Royal, that such a result is an experi- 

 mental proof of the rotundity of our satellite. A dispute has been going on 

 between photographers as to the proper angle for taking terrestrial stereo- 

 scopic pictures, and I infer that one side of the disputants would consider my 

 arrangement of moon-pictures to produce stereographs unnatural, because 

 under no circumstances could the moon itself be so seen by human eyes ; 

 but, to use Sir John Herschel's words, the view is such as would be seen by 

 a giant with eyes thousands of miles apart : after all, the stereoscope affords 

 such a view as we should get if we possessed a perfect model of the moon 

 and placed it at a suitable distance from the eyes, and we may be well 

 satisfied to possess such means of extending our knowledge respecting the 

 moon, by thus availing ourselves of the giant eyes of science. 



It does not follow as a matter of course that any two pictures of the moon 

 taken under different conditions of libration will make a true stereoscopic 

 picture ; so far from this being the case, a most distorted image would result, 

 unless attention be paid first to the selection of the lunar pictures, and then 

 to their position on the stereoscopic slide. It is possible to determine before- 

 hand, by calculation, the epochs at which the two photographs must be taken 

 in order to produce a stereoscopic picture ; but so many circumstances stand 

 in the way of celestial photography, that the better course is to take the 

 lunar photographs on every favourable occasion, and afterwards to group 

 such pictures as are known to be suitable. 



A little consideration of what has been before stated will show that two 

 lunar pictures, differing only by libration, either in longitude or in latitude, 

 will give a true stereoscopic effect, provided the angular shifting is suffi- 

 ciently great. 



On the other hand, if the two pictures differ both by libration in latitude 

 and in longitude, they will give a true stereoscopic picture provided they 

 satisfy the following condition. Suppose a point in the centre of the equator, 

 when the moon is in a mean state of libration, has become shifted at the 

 epoch of picture A in any given direction, and let an imaginary line pass 

 through that point and the centre of the lunar disc, if at the epoch of picture 

 B the point lies anywhere in the direction of that line, then a true stereo- 

 graph will be obtained, provided the two pictures be suitably placed in the 

 stereoscope. 



