PROCEEDINGS OF THE SOCIETY. 
31 
ways would give differences sufficient to account for the discrepancies 
observed. 
Mr. Ingpen wished to say a few words, by way of supplementing 
what he said after reading Mr. Wenham’s paper on his new method of 
measuring apertures. Since that time, he had seen Mr. Wenham’s 
way of doing it, and would endeavour to explain it more clearly than he 
had been able to do on the former occasion. In measuring the angle, 
Mr. Wenham used the object-glass without any object uj)on the stage, 
and having placed the instrument in a horizontal position, he marked 
off a circle having the objective in its centre ; he then put over the 
eye-piece an achromatic observing lens, and placing a liglit on a level 
with the objective, he turned the instrument round until the light 
began to disappear. Having measured the arc, and marked it as zero 
on the circle, he turned the instrument round in the opposite 
direction until the light began to fade on that side ; this so far was 
the ordinary way of measuring the angle. But then at this point he 
took a brass cap with a very minute hole in the centre, and put it 
over the eye-piece, and it was at once found that the small pencil of 
rays which could alone get through this hole was much less than 
those which got into the whole tube. He took a new yL-inch 
objective and tried to measure its angle in three ways ; in the old 
way it showed as 125°, then on looking at it with an observing lens 
it seemed about the same ; but the moment the cap was put on, down 
came the angle to 100°, and Mr. Wenham said that this was the true 
critical angle of the glass. The principle upon which the cap was 
used was then explained by means of black-board drawings, and it 
was shown that through the small hole they got the central pencil of 
rays which passed through the objective — not of course all that went 
through the front combination, but all that w’^ould pass through the 
lenses as observing rays. 
Mr. Slack asked if Mr. Ingpen had compared the readings 
obtained in the new way with those obtained by Mr. Wenham’s 
method. 
Mr. Ingpen said he had not yet had time to do this, having only 
received the apparatus on the previous evening ; but he had found 
that it did reduce the angle, although he could not say to what 
extent. 
Mr. Stephenson, in reply to a question from the President, said 
that Professor Abbe had shown him the apparatus when he was in 
London, at the Exhibition of Scientific Instruments at South Kensing- 
ton. Professor Abbe on that occasion tried several objectives; the 
lowest of these was Zeiss’s No. 2 immersion, which gave 105°, No. 3 
gave about 102°. It seemed to him to be a very excellent way ; on 
the occasion to which he referred the slab of glass was rather 
different from the one now exhibited, being square instead of half 
circular. 
Mr. Slack asked Mr. Stephenson if the measures taken by his owm 
plan differed much from those obtained by Mr. Wenham’s. 
Mr. Stephenson said he had never tried Mr. Wenham’s method, 
his remarks referred entirely to Professor Abbe’s. 
