598 DR. G. W. ROYSTON-PIGOTT OY A SEARCHER EOR APLANATIC IMAGES. 
These coincidences of eidola with true focal images may in both cases equally delude the 
observer. 
The next question was the most favourable distribution of the elements of magnifying- 
power. According to a well-known optical principle, it seemed desirable to bend the 
rays by less sudden refractions. It is a peculiar result that when the incident and 
emergent pencils are equally bent so as to be equally inclined to the axis of an equi- 
convex lens, that then only is the aberration a minimum. 
The effects of different distribution of power are well shown by the following experi- 
ments, in both of which the same amplification was employed of 400 diameters. 
Experiment 4. — A miniature landscape formed by a convexo-plane lens focal 
length and aperture was examined with the one-eighth and an “A” eyepiece: 
axis horizontal and window open. 
Result . — Landscape dark and hazy, as seen in the microscope. 
The deficiency of light teas most remarkable . 
The same power (400) was now obtained with the half-inch objective and a D eyepiece. 
Experiment 5. — The miniature being formed as before by the small lens, the micro- 
scope was now again brought into operation on the minute image horizontally. 
Result . — Exquisite picture brilliantly lit up ; even the foliage glittering in the sun- 
light was sharp, clear, and decisive, so that the details of the garden picture were mar- 
vellously displayed. 
The difference appeared truly surprising as regards the two methods of obtaining the 
same magnifying-power, especially the increased light with diminished aperture. 
In both these cases the greatest pains were taken to properly adjust the index collars 
of the objectives for the finest possible definition of an uncovered object. 
A new fact had appeared highly suggestive of further inquiry. Accordingly, distribu- 
tion of power was now varied by employing differently constructed eye-lenses, especially 
“crossed lenses”*, and inserting, midway between the objective and eyepiece, convex lenses 
of great variety. It was now seen that these lenses, intermediately placed, developed 
an entirely new aberration of a negative kind f. It became important to decide 
* Crossed lenses, well known to give a minimum aberration having - the radii of their curved surfaces 
as 6 : 1. 
t It is convenient to define the aberration to be positive or negative, or the lens to be over- or undercorrected, 
by the simple fact that a convex lens causes the ex centrical rays to cross the axis at a point nearer the 
centre of the lens than the centrical rays, in which case, and in all analogous cases, it may be said that the 
lens is undercorrected and afflicted with a negative aberration. English objectives are now constructed on 
the principle of having the posterior sets overcorrected and the anterior undercorrected so skilfully as to destroy, 
by opposite errors nearly, the residuary aberration ; but the opinion may be hazarded that future combinations 
will yet be found which will completely throw into the shade the present powers of the microscope, when per- 
haps we shall be in a better position to attempt to determine the microscopical features of molecular life, at 
present probably beyond its grasp, as no single particle so small as the sixty -thousandth of an inch in diameter 
' can be clearly defined if isolated, until residuary error is very much reduced. 
It is to be regretted that the precise nature of the marvellous combinations invented by Professor Amici for 
