364 



that a star viewed through it appears absolutely stationary, its firm- 

 ness is such that a hard blow against the side merely produces a 

 scarcely perceptible momentary deflection. The object-glass is 8 

 inches in diameter, and has a sidereal focus of 12' 5 feet the dia- 

 meter of the moon's image in this focus being about ] '35 inch. 



The eyepiece was removed, and in its place the body of a small 

 camera was attached, so that the moon's image might fall upon the 

 ground glass or sensitive film in the usual manner. Much labour 

 had been saved me in finding the true actinic focus, by several pho- 

 tographers of Liverpool, who were working for some time on the 

 same subject when the British Association met in that city in 1854. 

 They found that the object-glass had been over-corrected for the 

 actinic rays the plate being required to be placed at a distance of 

 0-8 of an inch beyond the optical focus : a few experiments were 

 sufficient to enable me to verify this result. 



During the time above referred to, and frequently since, Mr. Hart- 

 nup had taken many hundreds of pictures with chemicals recom- 

 mended by various persons, but had not succeeded in obtaining a good 

 negative at all, and not even a positive with a less exposure than 

 from half a minute to a minute. As I succeeded in taking dense 

 negatives in about four seconds, with the temperature of the room 

 below freezing and the moon at a considerable distance from the 

 meridian, and as I attribute the greater sensitiveness which I ob- 

 tained to the great purity of the materials I employed, I think it 

 right to give, after the mechanical arrangements are described, an 

 account of the way in which these were prepared. 



The clockwork movement was only sufficient to follow the moon 

 approximately when on the meridian, but as the pictures were nearly 

 all taken when she was some distance past the meridian, and when 

 consequently the declination and atmospheric refraction were changing 

 rapidly, it was necessary, notwithstanding the short time required 

 to take the pictures, to correct for the imperfect motion of the 

 telescope. This was done by means of slow-motion screws attached 

 to the right ascension and declination circles, which are each 4 feet 

 in diameter. The finder had an eyepiece with a power of 200 applied 

 to it, having cross wires in its field. 



The modus operandi of taking the picture was as follows : The 

 telescope having been moved until the moon's image was in the centre 



