280 
PROCEEDINGS OF SOCIETIES. 
defect in the very simple mode of measuring magnifying powers 
which was proposed by Dr. Beale, and which consisted in placing a 
scale ruled for 1000th of an inch on the objective. On looking 
through the oblique glass plate, and placing the ruled scale at ten 
inches from it, then measuring how many divisions of the scale this 
known 1000th of an inch occupies ; the one divided by the other will 
immediately give the magnifying power. It seemed to be a much 
more simple method of ascertaining the point. Were there any 
optical reasons why the plan was not so correct in its results as 
Mr. Ingpen’s? 
Mr. Ingpen said Dr. Beale’s contrivance was very good, but there 
were two objections to it. The neutral tint glass was placed in such 
a position that it gave a different magnifying power from what would 
be seen, because the image then was formed outside ; in his plan the 
image was formed in the exact conjugate eye-piece and dynamometer. 
If we could trust Dr. Pearson, that image does not vary with different 
eye-pieces. The distortion of area in the Huyghenian eye-piece was 
so great that no certainty could be felt in measuring according to 
Dr. Beale’s plan. 
Mr. Brooke: You simply want the distance between the two 
parallel lines. 
Mr. Ingpen : The distance would vary greatly if those lines were 
only to occupy say a | of an inch in one case and \ an inch in the 
other. He meant to say that y^ y -gths of an inch would not be exactly 
double , 0 V 6 th of an inch, in consequence of the great distortion of 
area which takes place in the Huyghenian eye-piece. It could be 
very easily tested with a micrometer. Then the thing is where do you 
measure your ten inches from ? A very slight difference in distance 
would make a very great deal of difference in the accuracy of the 
measurement ; if the micrometer was not perfectly horizontal, and 
so on, you could hardly get satisfactory measurements except of one 
kind. He thought Dr. Beale’s plan hardly came up to the require- 
ments of a standard. He thought also that a standard of a moderately 
simple character fitted with a piece of mechanism which could be 
made very accurately would be of greater value than that. Mr. Ingpen 
then adduced other reasons why Dr. Beale’s contrivance was not likely 
to give perfectly accurate measurements, and as perfect accuracy was 
indispensable he believed that the method of measuring magnifying 
powers advocated in his paper would enable the microscopist to attain 
such a desirable result. 
A vote of thanks was passed to Mr. Ingpen for his communication. 
Mr. Slack read a paper “On the Structure of the Valves of Eupo- 
discus Argus and Isthmia enervis , showing that their silicious deposit 
conforms to the general plan of deposition in simpler forms.” 
In reply to a question from Mr. Wenham, Mr. Slack said he 
thought the “ eye spot ” to be a light image at the focus of the minute 
lenses. The rays from the new illuminator did not show it as well 
as when more direct light was used. 
Mr. Stewart said that his examinations of the markings of the 
Diatomacefe led him to a different conclusion as to their nature. He 
