120 
THE YOUNG SCIENTIST. 
tained with this system in an § objective 
(the highest power then made) for reasons 
apparent in the diagram. The excessive 
depth of curvature of the contact-surfaces 
of the front pair is unfavorable for the 
passage of the marginal rays ; the soft- 
ness of the flint-glass forming the 
first plane was also objectionable. In 
the year 1837 Mr. Lister gave Mr. Koss 
a diagram for an improved "eighth," 
having a triple front lens in the form 
shown in Fig. 15. By this the passage 
of extreme rays was facilitated; and in 
order to diminish the depth of curvature, 
a very dense glass w^as used, having a 
specific gravity of 4"351. Faraday's glass, 
having a density of 6 "4, had been previ- 
ously tried, but was abandoned on account 
of a difficulty in working it. The polished 
fluence in correcting the oblique pencils, 
or in producing fiatness of field, and may 
be a plane with an equally good or better 
result. "Eighths" of this form with 
angles of 80° were made, and remained 
unaltered till the year 1850, when larger 
apertures were called for, and Mr. Lister 
introduced the triple back lens. 
The necessity for this will be seen by 
the diagram in Fig. 15, which shows that 
the contact-surfaces of the back achrom- 
atic are too deep, thus giving great thick- 
ness to the lens and limiting its diameter ; 
dense flint would have remedied this to 
some extent ; but its liability to tarnish 
rendered its use in a pair objectionable. 
The highest density at this time known, 
quite free from this defect, was 3 '686. By 
means of the triple back, the final correc- 
surfaces of both these qualities of dense 
glass speedily became tarnished by expos- 
ure to the air ; and thus the dense flint con- 
cave could only be employed in a triple 
combination, that is, when cemented be- 
tween tw^o lenses of crown-glass : this form 
of front was kept a trade secret, and was 
not published in any work treating of the 
optics of the microscope. The front inci- 
dent surface of the flint of the middle pair 
was made concave in order to reduce the 
depth of the contact ; and for this reason 
only, as that surface has but little in- 
tions were rendered less abrupt, a greater 
portion of the marginal rays could be 
collected, and the ai:>ertureof an " eighth" 
was at once brought up to 130° or more. 
At this time the author (Mr. Wenham) 
had been making some experiments in 
the construction of an object-glass in the 
form of Fig. 15. Mr. Lister having favored 
his "eighth" with an examination, w^as 
good enough to communicate his late im- 
provement of the triple back. No time 
was lost in giving this a trial, the result 
of which proved that excessive negative 
