71 
in the usual way on the ground glass. The chemical focus 
of the object glass was found to be eight-tenths of an inch 
beyond the optical focus, being over-corrected for the actinic 
rays. Although a good clock movement driven by water 
power is applied to the telescope, it was found necessary to 
follow the moon’s motions by means of the slow-motion 
handles attached to the right ascension and declination 
circles, and this was effected by using an eyepiece, with a 
power of 200 on the finder, keeping the cross-wires steadily 
on one spot. With this instrument Mr. Hartnup had taken 
a large number of negatives, but owing to the long exposure 
required he was not successful ; but with more suitable col- 
lodion and chemical solutions, and although the temperature 
of the observatory was below the freezing point, Mr. Crookes 
obtained dense negatives in about four seconds. Mr. Crookes 
afterwards enlarged his negatives twenty diameters, and he 
expresses his opinion that the magnifying should be con- 
ducted simultaneously with the photography by having a 
proper arrangement of lenses, so as to throw an enlarged 
image of the moon at once on the collodion plate ; and he 
states that the want of light could be no objection, as an ex- 
posure of from two to ten minutes would not be “ too severe 
a tax upon a steady and skilful hand and eye.” 
In an appendix to his paper Mr. Crookes gives some par- 
ticulars as to the time required to obtain negatives of the 
moon with different telescopes, from which it appears that 
the time varied from six minutes to six seconds. The different 
results named must, I conclude, have been caused not so 
much by the differences in the instruments as in the various 
processes employed, and in the manipulation. I must ob- 
serve, also, that it is not stated whether all the experiments 
were tried upon the full moon, a point materially affecting 
the time. 
' Mr. Grubb read a paper on this subject before the Dublin 
Photographic Society on May 6th, 1857. After referring to 
