206 
But as the process of taking single pictures is tedious, and 
as the area of the plate covered by the image is limited, it 
is possible to take four or more pictures on one plate. This 
little piece of apparatus is arranged for taking four pictures 
on a 5x4 plate. The great advantage of this plan of 
proceeding is that four different exposures may be obtained 
in very little more time than is required i.o take one ; and 
this is important, as it enables the operator to select the 
one which appears nearest to being correct as a guide for 
subsequent work on the same evening. 
With all telescopes, excepting reflectors, there is the 
difficulty of the chemical focus to overcome, and many 
failures arise in correcting this. 
The only advantage in using a telescope in photographing 
the moon, is that the length of focus gives a large image ; 
but for photographing the stars the field is too limited, and 
the time of exposure is of course long. It was in order to 
obtain a large field that I used a camera and photographic 
lens in photographing the eclipse in 1870. This method 
has been followed on every occasion of an eclipse since ; 
and I am glad to notice that in the very important work now 
being carried on in various observatories for photographing 
the stars the same plan is adopted. The introduction of 
gelatine plates has greatly facilitated the work of photo- 
graphing the stars and other faint objects, but so far as my 
own observation extends, the whole of the best eclipse work 
and the photographs of the moon have been done by the old 
collodion process. 
The discussion on Dr. Burghardt’s Paper “ On the Pollu- 
tion of the Biver Irwell and its Tributaries,” was resumed 
and continued by Mr. C. Trapp, Mr. Geimshaw, Mr. CoebetTj 
Mr. Wm. Thomson, and Mr. J. H. Kidd, of Wrexham. 
