168 SECTION PHYSICS. 



effects due to libration in longitude and latitude, and to parallactic 

 libration in shifting the crater in question from the position it occupies 

 in reference to the lunar disc at the period of mean libration. The 

 telescope with which these negatives were taken is now at Oxford, and, 

 I believe, Professor Pritchard, the Director of the University Obser- 

 vatory at Oxford, intends to devote the telescope to determining this 

 problem, and a very interesting one it is. I said that the size of the 

 negative depends essentially on the length of the telescope and the 

 position of the moon in her orbit. I have here a model of the very 

 splendid instrument, constructed for Melbourne by Messrs. Grubb, 

 of Dublin, under the superintendence of a Committee of the Royal 

 Society. In that colony they are more public spirited than we are here, 

 and they have found the money for constructing this very large and 

 perfect equatorial telescope, the mirror of which is four feet in diame- 

 ter, whereas the telescope, with which my photographs were obtained, 

 is only thirteen inches. Here, through the kindness of Mr. Ellery, 

 the Melbourne Astronomer, are some negatives of the moon three 

 inches in diameter, made with the Melbourne telescope, and very 

 beautiful productions they are. These especially would be particularly 

 well adapted for measurement with the micrometer for determining 

 the question of the libration, but even pictures so small as those 

 obtained with a thirteen inch reflector may be magnified to a suit- 

 able size. There is one on the wall thirty-eight inches in diameter 

 obtained in a negative taken at Cranford, and here is another, about 

 eighteen inches from diameter, a very beautiful one, taken by a Mr. 

 Rutherfurd, an American gentleman, in his private observatory. I do 

 not know whether that is his best, but there is one he has taken 

 under particularly fine conditions of the atmosphere, which is the 

 finest in existence. 



It may be interesting to you to know, as all the necessary apparatus 

 is before you, in what way the enlarged photographs are obtained. 

 One of these original negatives is placed in the focus of this long 

 camera. Lenses, specially made by Mr. Dallmeyer, are used for 

 projecting the image along the axis of the camera on to plates about 

 one-foot square, and then the first enlargement takes place. Here are 

 specimens taken from downstairs, this is a first enlargement to nine 

 inches in diameter, and is a transparent positive on glass ; by a second 



