264 On the Advancing Aplanatic Power [Mon Noe eo. 
ignored; yet I have no hesitation in saying that without this test 
it would be impossible to construct perfect objectives.” . . . “ This 
test discovers the least fault”—we all admit that a minute globule 
of mercury, say the 45th or even the 5;/55th of an inch diameter, 
displays a beautiful little image. Its size is easily calculated, which 
is done at Mr. Wenham’s request. 
The question now is, Why is the microscope incapable of de- 
fining the shape of this reflected image? Two reasons may be 
assigned—either it is imperfectly defined or formed in the focus of 
the objective, or else it is too diminutive to be distinguished. ‘The 
latter can be obviated by using a larger globule; the former arises 
either from the errors of reflexion from the surface of the mercury, 
or from errors in the microscope itself. 
As for the size, it will be shown further on that for minute 
globules the image is within a thousand millionth of an inch, the 
twentieth part of the globular diameter, provided the olyect 1s 
15 inches from tt, and 3 inches in diameter. 
Now for the other causes. The image formed by the globule is 
sufficiently clear of aberration to be distinguished by a perfect glass. 
The aberration is much more easily calculated than even for a re- 
fracting lens, and diminishes as the diameter lessens. . 
In order to introduce the subject in a clearer manner, I beg to 
describe an experiment exhibited at the soirée of the Royal Society. 
Having observed that plate glass contains air or other gas 
bubbles of many different sizes and degrees of perfection, and that 
these lenses possess an aperture of close upon 180°, I found that 
they form brilliant minute pictures as in a jet-black frame. You 
can with a Coddington see a whole prospect portrayed within their 
tiny black rings; the image being formed nearly at the back surface 
of the bubble. 
Placing one of these minute negative lenses on the stage, and 
reflecting the light directly, the flame, brass rim of the mirror, 
and other details, may be charmingly seen within it, with a low- 
power objective and a deep eye-piece. But a large-aperture 
objective and a low-power eye-piece, giving similar power on the 
whole, fails to give the same picture in its beautiful precision of 
definition. ‘The instant the aperture is reduced by means of the 
aberrameter, the picture regains its beauty and decision. 
This happens with the smallest bubbles, with minute solid 
fused glass spheres, and with minute plano-convex lenses. The 
effects of this kind of aberration are instantly displayed. If a 
brilliant flame be imaged in one of these very minute lenses, the 
shape of the flame is lost; it becomes around disk, but much larger 
than it ought to be. Detached single spherules from scales and 
diatoms show the same thing—spurious disks of refracted points 
of light. 
