175 
Experiment 4.—A miniature landscape formed by aconvexo- 
plane lens ;|; focal length and ;3, 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 micro- 
scope. 
The deficiency of light was 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 microscope was now again brought into 
operation on the minute image horizontally. 
Result-—Exquisite picture brilliantly lit up; even the 
foliage glittering in the sunlight was sharp, clear, and de- 
cisive, 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, distribution 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.” It became important to 
1 Crossed lenses, well known to give a minimum aberration having the 
radii of their curved surfaces as 6: 1. 
? 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 excentrical rays to cross the axis at a point nearer the centre of 
the Jens 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 micro- 
scope, when perhaps we shall be in a better condition to attempt to determine 
the microscopical features of molecular life, at present probably beyond its 
grasp, as no single particle so small as the sizty-thousandth of an inch in 
cioter 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 combina- 
tions invented by Professor Amici for objectives remain unknown. As one 
of the Jurors in the Paris Exposition, his microscope necessarily remained 
both uncelebrated and unelucidated in the Reports. 
