80 
If the chemical and visual foci are not coincident the image 
will have a blurred appearance. Before exposing the next 
plate, turn the adjusting screw so as to lengthen the tube 
about the 16th of an inch, and so proceed until, by the 
greater distinctness of the image, it is seen that the chemical 
focus is found. At every change of the focus a slight mark 
should be made on the tube, and when the true focus is satis- 
factorily determined the marks should be made distinctly 
visible ; and in ail future experiments with the same instru- 
ment the focus will be always at or very near the same place. 
Should it be found that the indistinctness increases, it will of 
course be necessary to try in the other direction. 
The appearances arising from atmospheric disturbances are 
very much the same as when the object is out of focus : expe- 
rience alone will enable the operator to determine from which 
cause the defect proceeds. 
It is assumed that the telescope is provided with a driving 
clock ; when such is the case every care should be taken that 
all the parts are clean, and when necessary oiled or greased, 
so that the motions may be as smooth as possible. 
In photographs of the moon in the phases, prior to and after 
the full, the side opposite to the sun is always too light, or 
“ burnt up,” while those parts near the terminator are often 
so dark that only the tops of the craters and peaks are visible, 
although in the telescope a clear and bright image can be 
seen. The cause of this must be that the exposure, if con- 
tinued long enough to bring out all the eye can see on the 
darker side, would entirely obliterate the details on the 
brightly illuminated portions of the moon’s surface. Mr. 
De la Rue’s suggestion as to why the dark side of the moon 
has so little actinic effect, has been already referred to. I 
would suggest that, as the light of the full moon is 100,000 
times weaker than that of the sun, the twilight * on the moon’s 
* In the absence of an atmosphere on the moon there can be no twilight 
such as we hare on the earth. The meaning, therefore, of this sentence may 
