THE LIMITING DIAPHRAGM, OR APERTURE SHUTTER. 13 

THE LIMITING DIAPHRAGM, OR APERTURE 
Siti ERS 
OR many years past we have heard declamations against the 
practical value of wide aperture objectives, principally how- 
ever, upon the ground that they do not possess penetration, though 
all seem agreed that they do define much better than those of 
small or medium angle. Even Dr. Carpenter, the champion of 
medium angles, in the last edition of his work, p. 732, seems at 
last to prefer wide apertures, for in the only instance in which he 
has expressed an opinion upon the structure of insect scales from 
iis own study he writes, “The author has fully satisfied himself by 
“his own study under an oil immersion 1-25th of Messrs. Powell 
“and Lealand of a Fodura scale illuminated by the ‘immersion 
“ paraboloid,’ ” etc., ete. 
It is well-known that the wider the aperture of the objective, the 
less the working distance may be, though this does not depend 
upon the aperture alone. A reference to pp. 257, 282, and 283 of 
this Journal will show this clearly, if columns a, d@, and g are com- 
pared ; we do not, however, wish to enter into this question at 
present, but will endeavour to show how penetration may be given 
to wide angle lenses. 
It seems upon inquiry that some of our opticians have for some 
time past produced a half-inch objective for use with the binocular, 
by placing a diaphragm of smaller aperture than usual behind the 
back lens of the objective. The half-inch of 60° is thus easily re- 
duced to 40°, and the penetration consequent upon such reduction 
is by this means obtained. 
For some time past we had been using stops of blackened card- 
board, and as these were not very convenient in use, we had a con- 
versation with Mr. J. B. Dancer of Manchester, as to the utility of 
making an “iris” diaphragm for the purpose, when he produced a | 
graduating diaphragm made ten years ago, but as to the best of 
our knowledge, our idea has never been published, we give a 
sketch of the instrument as made for us by Messrs. Ross & Co. 
Fig. 2 shows the instrument as a nose-piece screwing into the 
lower end of the microscope tube, while fig. 3 exhibits the iris 
diaphragm placed immediately above the objective, the lever which 
actuates it being shown in both figures. It is plainly a very simple 
piece of apparatus, but to be perfect, the aperture in the diaphragm 
should be capable of accurate centering. 
It will be seen that this form of aperture shutter enables the 

