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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, &c., as now applied to an object-glass, 
appear to have no definite meaning, but depend on the 
caprice of the maker, since the magnifying power of 
glasses, with the same fraction assigned to them, differs 
enormously. 
To return to Dr. Royston Pigott. He found the usual 
means of testing an object-glass by trying if it gave some 
particular appearance with a “test object,” such as the Po- 
dura-scale, very unsatisfactory, 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 as a watch-face or ther- 
mometer-scale, forming these images by aid of mercurial 
globules and the condenser properly adjusted below the 
microscope-field. By this means he has found that object- 
glasses corrected so as to show dark, sharply marked spines 
(like !!!) on the Podura-scale—a favourite test-object with 
our microscope-makers—give false, blurred, and distorted 
appearances with his known 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 at all as 
supposed, but is beset with a series of bead-markings, which 
by intersection, when improperly defined, give the curious 
appearance like notes of exclamation.* This important dis- 
covery of the falsity of our high powers (1-8th to 1-16th), 
has led Dr. Royston Pigott to pay more attention to the lower 
powers, and he finds that, though you may not get so much 
actual amplification, you yet get a truer effect, and greater 
clearness of detail, by employing very carefully made low 
powers (1-2nd to 1-5th), and increasing the magnifying power 
at the other end of the microscope, 7. e. the eye-piece. We 
lave in this way seen the beaded structure of the scales of 
Podura more satisfactorily than with very high objectives, 
even when corrected so far as they would admit, and we may 
say the same of some Diatom-valves, e.g., Pl. formosum. 
It would be most important to know how far such a change 
of combination would be useful in histological work. 
The general upshot of Dr. Royston Pigott’s investigations 
appears to be that it is desirable to shift the burthen hitherto 
cast almost wholly upon the objective to the other parts of 
the instrument. We should be content with an objective as 
high as a fifth, or even less. A very deep eye-piece is to be 
* See Plate VIII of this number, 
