ACHROMATIC COMPOUND MICROSCOPES. 
193 
The triple object-glass is thus very superior to a double one when each is 
used singly, and the union of two triple ones has been already proved in Eng- 
land to be eminently effective. Tulley’s 0.9 inch glass singly admits a pencil 
of near 20°, and his combination, one of 38°. 
These triple glasses and the double ones of Professor Amici are adapted by 
the form of their curves each to its respective place ; but the foreign double 
glasses which have their flint lens plano-concave, and particularly those of 
Utzschneider before mentioned, are made much on one model, and intended 
to be each good alone. It might seem but reasonable to infer from this, that 
they would be unfit to be combined ; and accordingly when screwed together, 
most of the numerous practicable changes of his series of five glasses, of which 
as many as four may be united for use, have much indistinctness from sphe- 
rical error ; and this is I think the case with all those combinations which the 
maker contemplated. Some peculiarities, however, observed two years ago 
in Chevalier’s object-glasses, led me to undertake a close examination of these, 
which were liberally placed at my disposal for the purpose, in the hope of dis- 
covering the cause for a discrepancy which appeared in their effects. 
I found that with a part of the combinations, the image of any bright point 
that was at some distance from the centre of the field had a faint light or coma 
stretching outwards from it ; with others the coma was as much inwards. 
The spherical aberration was in general much over-corrected, but in some 
triple and quadruple combinations the opposite error showed itself ; and out of 
the whole number one triple and one quadruple were remarkably beautiful and 
distinct. 
The result of this investigation was to disclose or confirm to me the exist- 
ence of some properties of the double object-glass, which have not I believe 
been hitherto recorded, and which it is now my purpose to describe only in 
connection with the subject before us, — the improvement of the microscope. 
With this in view I would premise that the plano-concave form for the cor- 
recting flint lens, which was probably adopted at first for its simplicity, has in 
that quality a strong recommendation ; particularly as it obviates the danger 
of error which otherwise exists in centering the two curves, and thereby admits 
of correct workmanship for a shorter focus. To cement together also the two 
lenses of the glass, diminishes by very nearly half, the loss of light from reflexion, 
2 c 
MDCCCXXX. 
