ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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them being a modification of Grubb’s method of determining the focal 
length of lenses. It was much larger than Prof. Thompson’s apparatus, 
being adapted to test lenses of G in. diameter and 30 in. focal length, 
and was about 5 ft. long and the same height. It proved very satisfac- 
tory in use. He regretted that the word astigmatism had remained on 
the prospectus and was kept in use, for it undoubtedly was wrong, but 
still it was a very convenient term to employ, and he did not know what 
could be substituted for it, and it was apparently understood by users of 
photographic lenses. There were other terms as regarded distortion and 
dispersion, which were not altogether correct, and should give place to a 
more accurate terminology. The point with regard to focal planes, 
which formed such an important feature in microscopic work, had not 
come before them in connection with photographic lenses, nor did they 
use it at all in testing telescopes and binoculars. 
Mr. T. R. Dallmeyer said it was very gratifying to find that at last 
the efforts which were being made to obtain perfection in optical work 
were to be subjected to critical and scientific examination, which must 
result in really good work being appreciated. He was particularly 
interested in what he understood to bo a new theory in treating light, as 
explained in the early part of the paper, and he wished Prof. Thompson 
had developed it further. One of the mail* things which opticians had 
to do was, after calculation, to do what he might describe as grinding a 
lens on paper, and this work, by ordinary processes, was very laborious. 
He imagined that if the process of which Prof. Thompson had only given 
the brilliant idea, were carried out with regard to the central pencils, 
that process of going through the mill in lens-grinding would be greatly 
facilitated. From the one or two hints which had been given, the 
method seemed to be simplicity itself. There were one or two other 
methods of obtaining the nodal points besides those which had been 
mentioned. He should like to ask if, in particular constructions — one of 
which he was deeply interested in — one of the nodal points was only 
radially outside the lens, would such an application be suited to the 
measurement of the lens as regarded its focus. He understood that, for 
taking long measurements, such an instrument would be hardly ajipli- 
cable, but that it was chiefly confined to the measurement of lenses where 
the nodal points were either contained in the instrument, or were very 
close to it. 
Mr. Conrad Beck said he also had been extremely interested in the 
paper, because he remembered his fearful struggles in endeavouring to 
work out lenses on the English system, and the delight with which he 
hailed the Gauss and the German system of reckoning both the signs and 
the principal planes. Those who had endeavoured practically to work 
out lenses on the ordinary system, as given in “ Parkinson,” and such 
books, would agree with him as to the enormous difficulties there were, 
which were entirely got rid of when the proper geometrical method of 
reckoning the signs, and the complete theory of the Gauss points were 
brought into work. As Mr. Whipple had said, the chief importance of 
the nodal points was with reference to microscopic work, and in that 
case there were some very awkward and difficult considerations. In old 
days, when the achromatic Microscope was first introduced, it was 
understood that some sort of reasonable magnifying scale should be 
