ASTRONOMICAL PHOTOGRAPHY. 167 



the moon. I have here a daguerreotype, copied from one which was 

 exhibited by the late Mr. Bond, of Cambridge, Massachusetts, in the 

 International Exhibition of 1851. 



You are aware that the moon does not always present precisely the 

 same hemisphere towards us in consequence of her unequal angular 

 motion in her orbit, while her axial motion, her rotation on her axis, is 

 perfectly equable. Although they coincide for a whole revolution, and 

 bring a crater of the moon exactly again to the same place at other 

 times we see a little on one side and a little on the other side of the 

 hemisphere which is turned away from us. Moreover, the moon's axis 

 is inclined to the earth's equator, and, consequently, sometimes we see 

 a little more to the north, and sometimes a little more to the south, 

 of the moon's equator as she moves round in her orbit. These effects 

 are called libration in longitude and libration in latitude, and we also 

 have the diurnal or parallactic libration dependent on the position of 

 the observer. The moon may be in the horizon or overhead, at one time 

 or the other, and in consequence of parallax, which is quite sensible, 

 being rather more than a degree, we sometimes see a little on one side 

 or the other of the moon in varying directions. We have here those 

 conditions which enable us to obtain a stereoscopic view of the moon, 

 by combining two pictures taken at different times, showing the moon 

 as a globe. But besides these three librations, it has been suggested 

 that the moon has a real or physical libration, in consequence, as 

 it is supposed, of a protuberance of matter on that hemisphere which 

 is turned towards the earth which would tend to follow the redundance 

 of matter about the earth's equator, and to adjust itself towards it with 

 a sort of balancing or wobbling motion. Whether there is or not a 

 physical libration of the moon, is a problem which astronomical photo- 

 graphy can solve, and I do not know that there is any other method 

 by which it could be done so perfectly. For this object the original 

 negatives may be placed on an instrument called a micrometer, and 

 adjusted concentrically with it ; then by bringing any object, a selected 

 crater for example, under the microscope, which is effected by turning 

 the divided circle, and then drawing out the slide, we can measure the 

 angular position and the distance from the centre or periphery of the 

 moon of that object, and obtain data for ascertaining whether there is 

 a physical libration of the moon or not, after taking into account the 



