PROCEEDINGS OF SOCIETIES. 
255 
Mr. Wenham’s paper, showing its connection with that which ap- 
peared in the November number of the ‘ M. M. J.,’ and explaining 
the difference between Mr. Wenham’s methods of measuring angular 
aperture, and those usually advocated. He also drew attention to the 
bearing of the subject upon illumination generally, the advisability of 
ascertaining, by means similar to those employed by Mr. Wenham, 
the exact optical conditions under which objects were examined, and 
the necessity for excluding all stray light, and for controlling the 
angle and direction of the illuminating pencils, in order to obtain fine 
definition. He further observed that the conditions of illumination 
suggested by Mr. Wenham’s papers as essential to good definition 
were in a great measure fulfilled in the case of a minute globule of 
mercury placed in the centre of the field of view and illuminated by 
reflected light. There was then no stray light from the rest of the 
field, and, whilst all the illuminating rays proceeded from the focal 
point, every part of the objective was in use that was capable of 
transmitting “ image-forming rays.” 
The President said the whole question was a very important 
one, because it was really of very little use to examine minute 
objects unless they were sure whether they were seeing real things 
or not. 
Mr. H. J. Slack said it seemed clear that any rays which reached 
the eye without passing through the focal point did not help to form 
an accurate image. He was not prepared to pronounce any decided 
opinion upon the whole subject, but he knew from experience that 
objectives which had a low angular aperture were capable of show- 
ing minute objects usually supposed to require large angles. One 
corollary from the doctrine of the paper was very extraordinary, none 
of their high-angled objectives being nearly what they professed to 
be, if Mr. Wenham was right. In one case one of 170° turned out, by 
his measurement, to be only 100°; and if he was correct, nobody had 
ever seen an object properly with any of the enormous angles supposed 
to operate. 
Mr. Charles Brooke confessed that without further consideration, 
and tracing all the rays through their courses, he could not clearly 
trace the exact justice of Mr. Wenham’s method ; but it was quite 
clear to him that the ordinary method of measuring the angle of an 
objective was fallacious. lie then drew upon the black-board a 
diagram showing the common method of ascertaining the angle by 
means of a candle flame, and pointed out that, although a distinct 
image could be formed at a wido angle on either side, if the object 
viewed was at some distance from the objective, it did not at all 
follow that at a nearer point on the same axis of vision for the 
distant object, they would also get the same distinct image of a 
near object. How far the method now proposed by Mr. Wenham 
counteracted that error, he was hardly prepared to say without 
further examination. 
The President said ho had listened to the paper and to Mr. Ingpen’s 
explanation with great interest, but he quite felt with Mr. Brooke that 
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