PROCEEDINGS OF SOCIETIES. 
159 
tissue investigations. [As a method for testing penetration (hitherto 
a desideratum), Dr. Morris proposes to examine a cover ground so as 
to be i^-^th of an inch thinner on one edge than its opposite, and to 
measure with an eye-piece micrometer the breadth of the band of 
ground glass distinctly visible (flatness of field being presupposed)]. 
Again, resolving power is the property of showing certain lines, 
markings, or shadows on diatoms, &c., and may or may not coexist 
wdth best defining power ; it depends in great measure upon angle of 
aperture, and is to a great extent an antagonistic property to penetra- 
tion. The dependence of resolving power upon angle of aperture is 
very well shown by placing a Pleurosigma angulatum, for instance, 
under a ith or -iQih, with such an eye-piece as will amplify suffi- 
ciently, and putting the compound body horizontally in front of a 
direct light. In this position no lines will be seen, but by rotating 
the compound body on its axis an oblique light is obtained, which at 
different angles, according to the power of the objective, will bring 
out transverse or oblique lines, and finally dots appearing as hexagons. 
The following results were thus obtained : 
Power employed. 
Trans- 
verse 
Lines. 
Oblique 
Lines. 
Hex- 
agons. 
ning to show at 25°) 
Spencer's |-inch, 0 eye-piece 
„ i-inch, 0 eye-piece 
Tolles' T^g-th, immersion (lines broken into\ 
dots) / 
Wales' aVth, immersion 
Powell and Lealand's -g^^th, dry 
Tolles' -p-Trth, immersion 
25 
20 
25 
0 
15 
50 
40 
20 
0 
20 
0 
We found that the -gV^h of Tolles gave good results as to flatness 
and clearness of field, penetration, resolution, amplification, and 
working focus or distance. Its definition is only fair, as also its 
working angle of aperture, while as to achromatism there is much 
improvement to be desired, and in working focus and general useful- 
ness much might be gained by setting the front lens less deeply and 
reducing the brasswork of the face. We were, however, agreeably 
surprised by the facility with which it can be handled. 
The gVth of Powell and Lealand was not equal to the above in 
clearness of field, nor in, definition, nor in working focus. Its pene- 
tration was equal, as also was its amplification, but its angle of 
aperture was 14° less. 
The 2^g^th of Wales is a very superior lens, giving good defi- 
nition, resolution, and penetration, while its other qualities are very 
fair. 
The -Y^th. of Tolles, although constructed mainly for use with 
oblique light, showed itself a good lens, with direct central rays. 
