506 MICROSCOPY. 
Thirteen years later Mr. Lister introduced the triple-back, for 
the same objects as the triple front, it being composed of a double 
concave of very dense flint between a plano-convex and a double 
convex of crown. Thus more marginal rays were collected, and 
the aperture of a 4 raised to 130° or over. 
At that time Mr. Wenham, experimenting in the construction of 
objectives, discovered that excessive over correction or negative 
aberration was easily obtained with lenses of shallow contact 
curves, and that color correction was chiefly controlled by changes 
in the triple back, the rays passing through the concave flint of 
the triple front so nearly in the direction of its radii that great 
changes in its curvature possessed only feeble chromatic effects. 
This led him to introduce the now familiar single front of plano- 
convex crown glass which was long rejected by the leading opti- 
cians, but is now used by all of them. The first } constructed on 
this system possessed an angular aperture of 130°, and was suc- 
cessful at first attempt, the middle pair being neutral or nearly 
achromatic and the triple back happening to have a suitable excess 
of negative aberration or over correction for color. Some posi- 
tive spherical aberration remained, which was remedied by giving 
additional thickness to the front lens ; a correction now considered 
essential and requiring great delicacy, as a difference in thickness 
of .<4y inch will determine the quality between a good and an 
indifferent +4. 
The excessive depth of the contact surfaces of the middle pair 
was a remaining defect, it being so great that if not balsam Ce- 
mented, total reflection of the marginal rays would take place, and 
the angular aperture be reduced. Though the surfaces are oblit- 
erated by being cemented with balsam and the rays are thus en- 
abled to proceed, still an angle beyond that of total reflection, 
implies excessive and detrimental depth of curvature. Placing 
the flint in the form of a meniscus above a plano-convex crown 
was employed as a middle pair with some satisfaction. An at 
tempt was also made to obtain the whole chromatic correction with 
the biconcave flint of the back, the middle as well as the front 
“being a single uncorrected plano-convex of crown. Sufficient 
_ over correction was obtained by the back to balance both the other 
lenses, but the red and blue rays, for instance, had become 50 
widely separated in the front and middle lenses and between them 
_ when placed for aplanatic foci, that they could not be brought 
