LISTER S DISCOVERIES. 
9 
16. Oblique Illumination. When an object cannot be 
illuminated by a large angular pencil of light, nearly the same 
effect may be produced by employing very obliqne illumina- 
tion, provided the object-glass has a large angular aperture, so 
as to enable it to take in a very oblique pencil falling in a 
direction to be taken up by one side of the lens only. This may 
be illustrated by the well known fact, that the numerous crags 
and peaks of distant mountains are better distinguished by the 
oblique rays of the setting or rising sun, than by the more 
direct meridian rays ; and for the same reason astronomers can 
see and measure the mountains in the moon better when it 
is new^ or at the quarter^ than when it is full. 
17. leister’s Discoveries. Among those who have contrib- 
uted to the improvement of the compound achromatic micro- 
scope, first and foremost stands the name of Joseph Jackson 
Lister, Esq. This observer, in 1830, presented to the Eoyal 
Society a paper, in which he pointed out certain newly discov- 
ered properties of achromatic lenses, by taking advantage of 
which object-glasses could be constructed, consisting of three 
compound lenses, each having its aberrations more or less cor- 
rected, by whose combined action a very large angular pencil 
could be transmitted, and admitting at the same time, with ease 
and certainty, of entire correction over the whole field, perfect 
definition extending over an entirely flat field. 
So valuable were the discoveries of Mr. Lister, that his 
paper has formed the basis of all the improvements in the 
compound achromatic microscope which have been made up 
to the present time. Stimulated by Mr. Lister’s discoveries, 
practical opticians have carried the improvement of the micro- 
scope so far that theory itself seems to point to but little further 
improvement to be desired, and the compound achromatic 
microscope has reached a degree of perfection, hardly equaled 
by even the most recent improvements of the achromatic 
telescope. 
18. Tlie essential requisites of a good Objeet-Olass, for 
the compound microscope, as stated by Mr. Lister, are : First, 
the transmission of a large focal pencil, free from all aberration. 
Second : that the field of view should be flat and well defined 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
