Meats Mcroscepica’| ~—- Royal Microscopical Society. 303 
by stops in the achromatic condenser. But as many lights 
arranged about a new object disguise its appearance seen from one 
point of view, so do these incorrigible decussing rays of the ordinary 
achromatic condenser disguise by confused shadows real appear- 
ance. 
The upper strange figure of a double flewr de lis was observed 
in a French objective of fair quality. The black figure 8 is a 
common form. . 
Diagram IIT. only represents a vertical section: the previous 
one gives a complex representation of complete pencils. 
In Diagram, II. a vertical section of a refracted pencil is de- 
lineated converging at a short distance. The rays in a horizontal 
plane converge to {, the locus of greatest light and greatest con- 
fle 
fusion. 
This suggests the propriety of covermg up a portion of the 
concave mirror to improve definition. 
The aplanatic definition of the microscope can only be effected 
when all chromatic and spherical aberration is destroyed by suc- 
cessful compensations. 
The aplanatic searcher, by traversing the axis, enables the ob- 
server to apply, to some extent, new corrections of a higher order. 
The final pencil emanating from the objective becomes more or 
less convergent at will, the eye-piece remaining fixed, and produces 
new changes in colour combination and spherical compensation. 
It increases magnifying power and penetration, and enables us 
to use a low objective instead of a much higher one giving as much 
ower. 
: Deficiency of light is avoided by using aplanatic condensers, 
which waste none of the light in the diffused blare or burr of 
aberration—shown readily by the aberrameter on the table. The 
wasted luminous fog is gathered up to a point, and the light is 
greatly intensified. 
The searching lenses found to have the best effect are from 1} 
to } inch focal length—are formed of two pair of achromatic over- 
corrected glasses, flint and crown glass cemented traversing the 
axis, and only slightly changing focal vision, the eye-piece being 
fixed. Innumerable corrections correlative with the screw collar 
may be applied and selected by traversing the lenses in search of 
the finest effects in colour and sharpness. 
Frequently an inferior objective has exhibited a better perform- 
ance under its influence. A Gundlach ith, lent me by our Secre- 
tary, Mr. Slack, would not show the Angulatum without it. 
The traverse or adjusting movement of the Searcher introduced 
a new available correction of the objective. 
