PROCEEDINGS OF THE SOCIETY. 
165 
which was entitled “ On the Eesnlts of a Computation relating to Tolies’ 
^ Objective.” It was accompanied by photographs of the Professor’s 
computation and of his diagram of the objective, showing the lenses of 
which it was composed and the path of a ray through it. 
Professor Stokes, after disclaiming for his paper all the importance 
attached to it by the President, proceeded to read it with comments, 
illustrating it by reference to diagrams, and by drawings upon the 
black-board. 
Mr. Ingpen ventured to make a few remarks on the practical 
bearing of the paper they had just heard. The theory was evidently 
correct, and would not be doubted by anyone acquainted with the 
more advanced optics, but there was some practical difficulty in 
securing an illuminating pencil which could utilize the increased 
angle of aperture obtained by the new arrangement. This could not 
be done by any ordinary condenser, where the light impinged upon 
the under side of a flat surface, but it was effected by Professor 
Abbe’s immersion illuminator, and others constructed on similar 
principles. Another difficulty — one for the optician — was to con- 
struct such middle and back combinations for the objective as would 
utilize the large cone of rays entering the front hemisphere. This 
Professor Abbe had certainly achieved to the extent of 113°, which was 
a great advance on all previous apertures. 
Mr. J. W. Stephenson (Treasurer) said it was true that they were 
limited to an equivalent angle of 180° in air if they had a plane dry 
surface beneath the slide ; to get the full effect they must, of course, 
have some medium to connect the condenser with the balsamed object 
instead of air ; Professor Abbe had now devised an immersion con- 
denser having a balsam angle of 138°. 
Mr. Mayall, jun., said that Professor Abbe had not been the first to 
devise means of illumination far exceeding the limit that obtains when 
the base of the slide is flat and dry. A lens almost exactly similar 
to Professor Abbe’s was figured by Mr. Weuham in the ‘ Quarterly 
Journal ’ more than twenty years ago, and the purpose was the same, — 
to obtain extremely oblique illumination. Mr. Wenham suggested its 
use with the paraboloid. Then we had the reflex illuminator, that 
works well up to a moderate limit ; the immersion semi-cylinder that 
permits the rays to fall on the object, when mounted on the slide or 
ill balsam, at an inclination approximating to 90° in glass, and various 
forms of immersion prisms, among which Dr. Woodward’s is par- 
ticularly practical. He did not understand Mr. Ingpen’s difficulty in 
providing oblique rays of sufficient intensitj'- for practical use, as the 
difficulty he had found was to regulate the amount of the light, not its 
obliquity. For example : by blocking out the whole of that part of the 
illuminating pencil on a balsamed object that corresponds to the air 
pencil of 180°, and using only rays beyond this inclination, he had 
found, with objectives having balsam angle of 95° and upwards, the 
more difficult images were made more appreciable by the eye. It 
appeared to him it was the excess of angle beyond 82° in glass of the 
illuminating pencil that enables us to see the more difficult images ; 
and so it is with the aperture of the objective. In the demonstration 
