186 NATURAL HISTORY MISCELLANY. 
nack, on the continent, not satisfied with the old system of combi- 
nations for object-glasses, and not having the benefit of Lister's 
researches, have made excellent objectives on a totally different 
system, and during the last few years the last-named maker has a 
carried his system of ‘“‘ immersion lenses”to such a point of excel- a 
lence as really to surpass the best glasses on Lister’s system, in @ 
definition, penetration, working distance, and illumination. Those 
who do not admit the excellence of these objectives, whieh are 
now used by nearly all German histologists, have probably seen 
older glasses, made at a time when Hartnack had not reached his 
best. It is worth stating, now that the Parisian opticians ar 
inaccessible, that Gundlach of Berlin has succeeded in making 
excellent glasses of high power at astonishingly small prices, some 
of his 1-12ths and 1-16ths, immersion 1-16ths (so called), being 
admirable in their performance. They are not, however, equal to 
Hartnack’s glasses, which, though costing far less than what simi- 
lar English glasses cost, yet are more expensive than Gundlach’s- 
It is only fair to all parties concerned to state that the terms 
1-8th, 1-12th, 1-16th, etc., as now applied to an object-glass, appear 
to have no definite meaning, but depend on the caprice of 
maker, since the magnifying power of glasses, with the same frac- 
tion assigned to them, differs enormously. Eo 
_ To return to Dr. Royston Pigott. He found the usual means 
testing an object-glass by trying if it gave some particular appear- 
ance with a “test-object,” such as the Podura-scale, very unsatis- 
factory, since we have no certainty to begin with as to what is the 
true appearance of such an object. He therefore examined minute 
images of objects of which he knew the true form, such -o 
watch-face or thermometer-scale, forming these images by aid or 
mercurial globules and the condenser properly adjusted below the 
8 
microscope-field. By this means he has found that object-glasse* 
corrected so as to show dark, sharply marked spines (like!! 1) on 
the Podura-scale— a favorite test-object with our microscope-mak- 
ers — give false, blurred, and distorted appearances with his kno’ š 
images, and on making such corrections of the objective as to show 
the known images in their true form, he finds that the Podura- 
scale, examined with the corrected objective, is not really marked 7 
at all, as supposed, but is beset with a series of bead-marki"e” — 
which by intersection, when improperly defined, give the curious — 
appearance like notes of exclamation. This important discovery 
