MICROSCOPY. 505 
under a new obligation, by his last paper on object-glasses, con- 
tributed to the Royal Microscopical Society. He seems to be 
strangely unconscious of the fact that he cares more for human 
progress than for trade secrets; and he publishes, with the utmost 
apparent indifference, exactly what the world wants to know, but 
what it knows too well would be by most persons devoted to se- 
crecy and to personal business purposes. 
Introductory to an explanation of his new formula for objec- 
tives, Mr. Wenham reviews the history of the modern (English 
objectives, 
In the year 1829, before which time three superposed achromatic 
lenses were employed simply as a means of increasing power, the 
late Mr. Lister discovered and published the law of aplanatic foci, 
that by separating suitably connected lenses one or two positions 
could be found in which spherical aberration was balanced; and 
Mr. Ross constructed in 1831, with unexpected success, the first 
objective embodying this principle. Mr. Ross then discovered 
that the interposition of a cover-glass removed the aplanatic focus 
to a different plane, causing negative aberration and requiring the 
lenses to be brought closer together; and he therefore introduced 
the screw collar adjustment which has now become universal. 
These objectives consisted of three pairs, the double convex crown 
and plano-concave flint of each pair having their contact surfaces 
of equal radius and balsamed together, the three pairs having foci 
about in the proportion of one, two, three, and the anterior pair 
ing at a considerable and variable distance from the other two 
pairs. In this combination the softness of the flint glass forming 
the first plane surface was unfortunate, and the angular aperture 
of a } was limited to 60°. 
In the year 1837, Mr. Lister furnished Mr. Ross a diagram of 
a triple front lens, consisting of a plano-concave of flint between 
two plano-convex crowns, for the purpose of protecting the flint 
from the exposure to the air and of diminishing the depth of 
curvature, which was unfavorable for the passage of the marginal 
Tays. The front surface of the middle pair was made concave 
with no other advantage than reducing the depth of contact, and — 
it may be made a plane with at least equally good results in cor- 
rection of the oblique pencils and in flatness of field. An angle 
of 80° was attained, by this method, in one-eighths. 
