81 
surface must be very much less, and consequently the actinic 
effect of the light is lessened in the same wav as at a corre- 
sponding time on the earth. 
The question, then, of photographing the terminator is only 
one of time, and in order to remedy the defect spoken of to 
some extent, I have used diaphragms such as are shown in 
the drawing. In the tube E openings are made on opposite 
sides, and wide enough to admit the diaphragms to be used 
without touching the tube. The diaphragm must be of the 
proper length and width to shut off the moon’s light until the 
plate is ready for exposure. The shape of the diaphragm will 
suggest which form should be used according to the moon’s 
age. The exposure should be made with the full aperture 
for as many seconds as previous experiments have proved to 
be necessary for the bright side, and the diaphragm then 
gently moved and kept in motion, gradually approaching the 
darkened side. By this means the exposure may be regulated, 
and the great differences in the light and dark sides of the 
moon may be modified. 
As to the processes employed, each experimenter must 
adopt the one he finds in his hands gives the best result. It 
seldom happens that two operators can produce the same 
effects with, apparently , the same chemicals. Experience has 
shown me that the ordinary patent plate glass (carefully 
selected, so as to be free from scratches and other defects,) is 
preferable to the white patent plate, having found that after 
a time the surface becomes covered with a kind of dew or 
“ sweat,” as it is termed, owing to the decomposition of some 
of the salts used in the manufacture. The collodion used was 
made for me by Messrs. Huggon and Co. of Leeds ; it is very 
quick, free from structure, and suitable for iron development. 
be misunderstood. It was intended to say that on those parts of the moon, 
enlightened only by the oblique rays of the sun, the light is so diminished 
that the actinic effect is lessened as it is on the earth shortly before sunset 
and during twilight, when it is well known that a much longer time is 
required to obtain a photograph. 
