ON CELESTIAL PHOTOGRAPHY IN ENGLAND. 141 



after a lunar eclipse, or at all events except when the sun, earth, and moon 

 are very nearly in the same plane ; at all other periods of the full moon we 

 are unfavourably situated for seeing the whole of the illuminated hemisphere. 

 Moreover, the different apparent diameter of the moon at various times, 

 dependent on her distance from the earth, comes out in unmistakeable pro- 

 minence in a collection of photographs ; for the pictures taken with my reflector 

 vary in diameter from one inch to one inch and nearly two-tenths (1-0053 

 inch to 1-1718 inch, being at the moon's mean distance 1*0137 inch). 



When positive enlarged copies are made, it is easy to obtain all the 

 pictures of exactly the same dimensions by the adjustment of the distance 

 of the negative to be copied from the lens of the camera ; and my enlarging 

 camera is furnished with screws to facilitate the adjustment of the distance 

 of the object to be copied, and also that of the focusing screen. 



Libration. — We are familiar with the terms " diurnal libration," and 

 libration in " latitude " and " longitude," yet it is difficult to realize the 

 great amount of disturbance in the aspect of the moon's disc, and the 

 direction of the displacement from the mean position which these several 

 .causes produce unless aided by photography, when we see them palpably 

 before us. • 



The diurnal or parallactic libration never exceeds 1 VS ; the direction 

 of the displacement in the markings on the lunar disc which it produces is 

 variable, and is dependent partly on the position of the observer. 



The poles of the moon at the epoch of Mean Libration are situated in the 

 periphery, and the equator and all parallels of latitude are straight lines ; 

 the circles of longitude being more or less open ellipses, varying from a 

 straight line in the centre to a circle at the periphery. This occurs when 

 our satellite is either in perigee or apogee (when the libration in longitude 

 is at a minimum), and she is also situated in one of the nodes of her orbit 

 (when the libration in latitude vanishes) : the nodes, apsides, and moon 

 would, under these circumstances, be in the same line. 



Libration in Longitude merely causes a change of place in the various 

 circles of longitude, which still continue to be more or less open ellipses; 

 the parallels of latitude straight lines. 



Those lunar craters, however, situated on the central meridian at the 

 epoch of mean libration would be on a straight line, but, at the periods of 

 maximum eastern or western libration, they would be seen arranged on a 

 semi-ellipse, whose conjugate diameter is 0*1377, the moon's diameter being 

 unity. Therefore a point at the centre of the moon's equator becomes 

 shifted by the sum of the librations to the east and to the west to the extent 



