72 
the fact that he found the actinic focus of his object glass to 
be longer than the visual (thus agreeing with Mr. Crookes) 
he states it to be generally understood that in a compound 
object-glass made as nearly achromatic as possible, the actinic 
focus is shorter than the visual. The most valuable portion 
of Mr. Grubb’s paper is the suggestion for a piece of appa- 
ratus to be attached to the part connected with the telescope 
for holding the dark frame, which he proposes may be so 
arranged as to follow the moon’s 'motion in declination ; and 
he gives the following description of a contrivance used by 
Lord Rosse, and which is suitable for telescopes not equa- 
torially mounted : — “ On a flat surface attached to the teles- 
cope, and parallel to the plane of the image is attached a 
sliding plate, the slide being capable of adjustment to the 
direction of the moon’s path at the time of operating. The 
slide is actuated by a screw moved by clockwork, and having 
a governor or regulator of peculiar construction, which acts 
equally well in all positions. The clockwork being once 
adjusted requires no change ; but the inclination of the slide 
must be effected by trial for the moon’s path at the time of 
taking the photograph.” This idea originated with Mr. 
De la Rue, Lord Rosse’s share in it arose from his having 
applied a clock motion to the apparatus. 
The telescope used by Mr. Grubb is l&flo inches aperture 
and twenty feet focus, giving an image 2tV inches diameter 
in from ten to forty seconds. 
The next contribution on this subject is by Mr. Fry, who, 
in 1857, commenced his experiments on the moon with an 
equatorial telescope, the property of Mr. Howell, of Brighton. 
The object glass of this instrument is eight and a-half inches 
diameter and eleven feet focus, and gave an image of the full 
moon in about three seconds, but under very favourable 
circumstances a negative was made in a single second. The 
size of the image is not stated, but it must have been about 
one and a-quarter inches diameter. Mr. Fry appears to have 
