On Angle of Aperture. By H. J. Slack. 
235 
with B eye-piece if a little superfluous light is screened off by 
holding a sheet of white paper, so as to stop some rays from 
entering the field. With the diaphragm eye-piece, constructed by 
Mr. Eoss at the request of the writer, many years ago, a person 
acquainted with the object can just see the cross markings with 
A eye-piece. B eye-piece with diaphragms makes them quite 
plain. 
Powell and Lealand's -|th will show much more difficult lined 
objects with direct light, but P. hijojoocamjpus was selected to suit 
a range of powers and angles of aperture. 
Zeiss has worked, under the direction of Professor Abbe, of 
Jena, on a plan precisely opposite to that usually followed by our 
leading opticians at the request of their customers. He has, so to 
speak, minimized angles of aperture and secured great working 
distance and penetration, and yet obtained an amount of separating 
or resolving power hitherto supposed to be exclusively the property 
of far larger angled glasses. 
The experiments mentioned should be tried on a clear day, with 
the microscope some way from the window, where there is a little 
shade ; and it is well to surround the eye-piece with a screen of 
black cotton velvet, to keep all glare from the eye. Excess of 
light, whether from the sky or a lamp, affects small-angled glasses 
more detrimentally than larger ones ; and it was through not being 
aware of the amount of caution required in this respect that the 
writer underrated the corrections and powers of the Zeiss D, 
when describing its merits last June in a letter to the ' M. M. J.' 
By lamplight an angle of about 45° with central stop, or two radial 
slots, is best for the 0 objective, and an angle of from 55° to 75° 
for the D when an achromatic condenser is used for light-ground 
illumination. 
Many other diatoms are well shown by Zeiss* C and D objectives, 
with their small angles and ordinary illumination with sub-stage 
mirror or condenser ; but if we take a delicate valve of P. angu- 
latum that is not satisfactorily exhibited in this manner, we can 
instantly increase the resolving power by the employment of Mr. 
Wenham's dark-ground Eeflex Illuminator, provided the object 
adheres to the slide and not to the cover. With this apparatus 
and C and D eye-pieces, P. angulatum with the terminal lines of 
beads where fractures occur can be sharply and elegantly displayed 
with the D objective. So great is the resolving power of this 
process, that the writer was able, at the late Scientific Evening of 
this Society, to show the transverse marks of Surirella gemma 
resolved into beads with this objective (^th), and C and D eye- 
pieces of Eoss's series. It is not pretended that any glass yet made 
with so small an angle is the best for showing such objects, or can 
display them as well as those with larger angles. The use of such 
