390 
PKOCEEDINGS OF THE SOCIETY. 
the observation is made. There is necessarily a particular intensity 
the most favourable to enable us to distinguish these small differences. 
“ If we look at the sun with the naked eye, we do not see the spots, 
although from their size they should be seen. If we take a smoked 
glass we see them at once. With too dark a glass they disappear, 
although we may still see the disk of the sun. This phenomenon 
takes place with the microscope when the illumination is too brilliant 
or not sufficiently so.” 
The President said that Dr. Pigott, after the meeting, would 
exhibit his aj^paratus to the Fellows. 
Mr. F. H. Wenham read a paper “ On the Measurement of the 
Angle of Aperture of Objectives,” illustrating the subject by a dia- 
gram, enlarged upon the black-board by Mr. Stewart. Mr. Wenham 
also said that Mr. H. Davis, who was present, had seen the operation 
of measurement, and would vouch for the fact that it was a very 
simple affair. (Mr. Wenham’s i3aper will be found at j}. 821.) 
Mr. Henry Davis gave some extracts from a paper he was about 
to present “ On the Pygidium of Insects,” illustrating his observations 
by drawings on the black-board. (The paper will be printed in the 
next number of the Journal.) 
Mr. Charles Stewart thought that the term pygidium was merely 
the name given to a particular tergal plate of the posterior part of the 
abdomen, upon which, in the case of the flea, these curious hairs were 
found. He believed that somewhat similar hairs had been described 
by Dr. Braxted Hicks and others, as occurring on other parts of the 
bodies of insects. He only wished to ask, as a matter of information, 
whether the term was applied to the hairs, or whether it belonged to 
the particular part upon which they were ordinarily found ? The 
circi, as these things had been called to which Mr. Davis drew 
attention, were really a pair of upstanding bodies, and were each 
divided into twelve little pieces ; they were not at all unlike posterior 
antennm, and he thought they would hardly come within the descrip- 
tion of pygidia, although they might bear similar hairs, and perform 
a like function. 
Mr. Davis said that the term had always been applied to the organ 
itself in the case of the lace-wing fly, and therefore he had from 
its analogy applied it to the organs of other insects ; but if it was 
wrong to apply it to the fly, then it was wrong to apply it to the 
others. On the other hand, however, if this organ in the lace-wing 
fly was a pygidium, then that of the locust was certainly a pygidium 
also. In some of the species of cockroach, the same thing was found 
starting from the same place, but only carried out a little longer. As 
regarded the cockroach (Blaita), it might at first be thought that the 
thing was not the same, but he felt sure that if anyone went through 
them carefully and traced them out by their analogies, there would be 
no difficulty in showing that they were really the same. 
iVJ r. Stewart said that if he recollected rightly, at the posterior end of 
