Royal Microscopical Society. 
55 
II. — On the Invisibility of Minute Refracting Bodies caused by 
Excess of Aj)erture, and upon the Development of Black 
Aperture Test-Bands and Diffraction Rings. 
By Dr. Eoyston-Pigott, M.A., F.E.S., &c. 
{Read lefore the Eoyal Microscopical Society, Jan. 6, 1875.) 
The invisibility of minute bodies, subtending a sufficient visual 
angle to be readily seen if properly defined, is a bighly curious and 
important fact, to wbich our Hon. Secretary, Mr. Slack, bas already 
drawn the attention of the Koyal Microscopical Society. 
This invisibility depends upon several causes, which it is now 
intended to examine. 
Minute bodies are often solely distinguished by the sharpness 
and decision of their outline. The question is — can this outline be 
obliterated by the conditions of vision and by any relation between 
the refractive index of the substance and the aperture of the objec- 
tive employed ? A few experiments will now be related which may 
perhaps help to unravel these points. 
First starting with gas bubbles found in plate glass (probably 
nearly vacuum bubbles) — if glass of very fine quality be chosen, 
(having surfaces true enough to exhibit Newton's rings * under pres- 
sure,) and these be examined with a horizontal microscope placed 
opposite the window, a very perfect picture of the prospect will be 
seen in miniature, surrounded by a black band ; the field of view 
will be found precisely three-fifths of the diameter of the bubble, 
or the band one-fifth. The curious fact is, that for this hollow lens 
the breadth of the band is the same for all objectives, whatever be 
the aperture. 
Not so, however, with a solid spherule of the same size and of 
the same glass. Two conditions regulate this breadth. 
I. The band increases in breadth from nothing till it occupies 
the whole spherule as the aperture is dipainished. 
II. The degree of aperture at which this black band first 
appears varies with the refractive index of the bead. 
It results from these principles that the aperture of an objective 
regulates the appearance or disappearance of the circular black out- 
line of minute refracting spherules, and consequently the black 
bands of refracting cylinders. 
Having thus stated the points to be attended to in observing 
such bodies, I proceed to describe more in detail the facts from 
which these conclusions have been deduced. 
I. In the case of the air bubbles, in plate glass, care must be 
* Of course it is only when one of the surfaces is truly spherical that Newton's 
rings are developed : in nearly flat surfaces these rings take every imaginable 
shape under pressure and every variety of prismatic hue. 
