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IV. — On the Present Limits of Vision. 
By Dr. Eoyston-Pigott, M.A., F.C.P.S., M.KG.P., F.E.S., 
F.E.A.S. 
(Taken as read before the Rofal Microscopical Society, 1876.) 
The subject of minute vision, and tlie limits to minute investi- 
gation, assigned to the nature of light, to the powers of the 
organs of vision, and to the construction of optical instruments, is 
full of a deep and abiding interest. To this end, the powers 
of philosophers in all ages have been anxiously turned. The 
subject is doubtless one of extreme difficulty and delicacy. I 
must frankly own, that I believe this limit has not yet been 
reached. The reasons which may be given for this belief, may 
be conveniently arranged under the nature of light, the powers 
of the eye, and of the instrumentation. 
1. The Nature of Light — Vibration and Colour. 
If light consists of a vibrating medium, forming waves, it is 
argued that the wave principle necessarily precludes, under the 
most favourable conditions, the formation of a visible image less 
than half a wave-length. This theory, originally applied by 
La Grange, has been popularized by Helmholtz and Abbe. Sir 
Eichard Airy, in his undulatory theory, shows that a minute disk 
of light assumes a spurious diameter in proportion to the smallness 
of the aperture through which it is viewed. If, for instance, a 
brilliant luminous point be examined through a pin-hole perforated 
in a card, the disk of light is enlarged apparently in proportion as 
the aperture is reduced. In the same way in telescopes the disk 
of a star, always enormously spurious, is diminished as the linear 
aperture is increased ; whether the instrument be a reflector or 
refractor. And in the microscope the same thing is accomplished 
by diminishing the aperture by a stop placed immediately behind 
a compound achromatic objective. And various rings should be 
developed around the spurious disk in every case. 
These are observed facts, nicely agreeing with calculated 
results. All these results may be combined in one conclusion — 
the effective linear aperture of the pencil of rays presented to the 
eye of the observer by whatever instrumentation regulates the 
apparent diameter of the spurious disk and companion diffraction 
rings, while other circumstances modify their appearance, form, 
number, and prismatic tints, or even cause their obliteration. 
But in order to develope the spurious disk and companion 
rings, a brilhant point of light is required. These phenomena, 
observed by means of the microscope, are very similar to those seen 
