190 Transactions of the Boyal Microscopical Society. 
the interstices of the large beads, closely packed and nearly black, 
estimated at about 130,000 to the inch. 
I venture the prediction that the glories of the Podura scale 
will not be exhausted for many a long year to come. I await with 
hope the time when these apparent limits of vision shall be greatly 
surpassed. 
The same goodness of definition was displayed in the " dry 
mount " Formosum, the spaces between the spherules occupied by a 
different set: individual beads appear bright in the centre, sur- 
rounded by a very dark ring, and show a very perceptible bright 
focal point. A fine state of defining power appears to light up as 
it were the usually dark dull appearance of this dry object. 
The Kev. Mr. Dallinger's Plates of the flagella of certain 
monads developed in the decomposition of cod's head, render it 
probable that he has been able to distinguish threads of a much less 
diameter than the t5 uV^o of an inch, with a Powell and Lealand 
sVth, and very special precautions of illumination (see Plates to his 
article). 
The apparent size too of the smallest cilia in a variety of 
organic structures, evidently demonstrates that a much less dia- 
meter than the diffraction limit as given by Helmholtz for brilliant 
images is often reached by the practised microscopist. 
On the whole, I am led to conclude that although theoretically, 
and for brilliant lines and points, the separable interval may be for 
the widest aperture ha.lf a wave-length, yet when by proper 
precautions of illumination the diffraction can in a great measure 
be destroyed, with very accurate glasses the limit is much smaller 
than the one assigned. Even with the purple or rather the violet 
ray, the half wave-length for brilliant diffractions would be about 
T^^oVo 0 of an inch, but for cases of reduced or destroyed diffraction 
very much less. 
Whether, then, the nature of light, as producing spurious disks 
and rings, or developing colours in different points of the focal axis, 
or the powers of the eye be considered, I have reasons to believe 
that the limits of vision have not yet been reached. 
The diameter of the ocular pencil, indeed, for brilliant objects, 
regulates the size of the fringes inversely, which may efface con- 
tiguous images, but nevertheless, by means of small pencils, under 
special illumination, the eye is capable of descrying objects much 
smaller than the effacing fringes when skilfully reduced, if not 
destroyed. Manipulative skill, therefore, is still a great desideratum 
for the microscopical tyro who aspires to pass beyond present 
attainments. 
