APLANATIC OBJECT-GLASSES. 
11 
are called the aplanatic foci. When the longer aplanatic focus 
is used, the marginal rajs of an oblique pencil (from a point 
on one side of the axis) are distorted so that the objects seen in 
the borders of the field appear fringed with a coma extend- 
ing outwards, while the contrary effect of a coma directed 
towards the centre of the field is produced by the rays from the 
shorter focus. The correction of chromatic aberration, like 
that of the spherical, tends to excess in the marginal rays. 
21. Aplanatic Object-Glasses. These principles afibrd the 
means of destroying, with the utmost certainty, both aberrations 
in a large focal pencil, by combining two or more achromatic 
lenses in a single object-glass. The rays of light from an object 
are received by the anterior combination from its shorter 
aplanatic focus, and are transmitted to a second achromatic 
lens, of such form, and so placed, as to receive the rays in the 
direction of its longer aplanatic focus. If the lenses are fixed 
at this distance, the radiant point may be moved backward 
and forward, as required to increase or diminish the length of 
the microscope, without disturbing the balance of the correc- 
tions ; since the motion of the radiant point produces opposite 
and equivalent errors in the two compound lenses. Slight 
errors in color may be destroyed in the same manner by their 
opposites, and thus we not only acquire fine correction for the 
central ray, but all coma of oblique pencils is destroyed, and 
the whole field is rendered beautifully flat and distinct. 
In the application of Mr. Lister’s principles, in order to 
enlarge the angular aperture as much as possible, it is found 
better (as Mr. Lister himself suggested) to retain in the ante- 
rior combination a certain amount of positive aberration, to 
be corrected in the posterior combinations, the proportionate 
dimensions of which are somewhat varied to secure the best 
effect in the entire compound achromatic objective. Some- 
times, also, in objectives of high power, the anterior and poste- 
rior combinations are each made to consist of three lenses, 
while the middle combination has but two ; but each combi- 
nation is specially calculated for the place it is to occupy, and 
more or less corrected by itself, as is found best to secure the 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
