566 General Notes. [ September, 
partly for convenience in class use in teaching, and partly that they may, 
by the test of practical use, receive any necessary corrections or additions 
before the publication of the full work. 
Aperture or OBJECTIVES. — Mr. F. H. Wenham’s experiments with 
[ 
the slit as a means of cutting off the lateral rays of an objective have led 
him to announce the belief, in the Monthly Microscopical Journal, that an 
excessive or false aperture is attributed to all objectives by the lateral pen- 
cis which direct light far beyond the axial one, and thus greatly enlarge 
the diameter of the proper light disk. An aperture mapped out ona 
screen shows the false aperture faintly portrayed as an outer circle of 
light, while the true aperture, as obtained by the slit, gives a bright, oval 
disk within the other. As an example of the effect of the slit in reduc- 
ing to what he regards as the true aperture, he mentions the following 
reduction of the nominal angles of three lenses made nearly twenty years 
ago: a} of 100° to 56°, a } of 130° to 92°, and a yy of 170° to 100°. 
He invites discussion upon this novel and very interesting question, 
which ought to excite the greatest attention until settled beyond 
dispute. 
Mr. Wenham now uses a slit of fixed width, cut through an opaque 
film upon a glass slip 3x1, being substantially the method contrived by 
Mr. Tolles, and published in the Naturavist for March, 1875. He 
also adopts without credit Mr. Tolles’ plan of covering the slit with @ 
balsam-mounted thin cover-glass, so that the objective can be adjusted 
and tested under natural conditions. 
PHOTOGRAPHING THE NINETEENTH Banp.— Count Castracane has 
photographed, apparently successfully, Nobert’s nineteenth band, with an 
amplification of eight hundred diameters obtained by means of a Gund- 
lach dry lens of 4, German inch focus. The object was illuminated by 
an achromatic condenser of large angle, and with a large central stop. 
The resolution of this band of lines of qr}37y inch by a dry lens, has not 
been generally deemed possible heretofore. The genuineness 
photographic lines was established by micrometric measurement. : 
success, if reliable, seems an invasion of Helmholtz’s theory On the ulti- 
mate limits of microscopic power ; but such theories seldom live long: 
Porvutar Microscopy. — The increasing use of the microscope 
among persons of previously unscientific habits and educati 
treating the subject in books and journals. Mr. John Phin’s litt in 
on the Selection and Use of the Microscope, intended for beg 
and under the same management. 
ed 
Aw Easy Nrrzscura.— The No. 19 of Möller's test plate, resol 
