40 
Williams^ on the Martin Microscope. 
manufacture of these instruments^ but also the kind of work 
this particular microscope was intended to perform^ and not 
expect it in any way to rival our present microscopes. It was 
evidently intended chiefly for the exhibition of coarse objects_, 
the details of which could readily be made out with low 
powers. Such objects are shown tolerably satisfactorily in it. 
Neither must we imagine that the array of powers I have men- 
tionedj although they do range to gVth of an inch focal length 
with the compound body, will at all compare with our ^ths^ 
-l^ths, or -nrths, either in definition or magnifying power. 
ThuSj the TT, th, which ought, according to our present notions, 
to resolve all the ordinary tests, will not line even a common 
butterfly's scale, neither is the magnifying power such as might 
be expected from such a denomination. The higher powers, viz., 
the 15th, 20th, and 30th, line ordinary tests, but will not touch 
the more delicate — such as the fine lines on the small scales 
of the lepisma, or the markings on the podura, and, con- 
sequently would be entirely useless in the examination of the 
still more delicate diatomacese. I may here give the result of 
my examination of the three higher powers — the 15th, 20th, 
and 30th. The test I employed was the scale of Morpho 
Menelaus, which would not at the present time be considered 
as by any means a severe one. They were tried by candle- 
light. 
The 15th. — Indistinct traces of lines very ill defined. 
The 20th. — Lined distinctly, but the definition anything 
but sharp. 
The 30th. — Lined distinctly but still without good defi- 
nition; the amplification in all these cases being far beyond that 
which is necessary for bringing out the lines sharply with the 
present object-glasses of much lower power. 
The very finely lined scales of the lepisma were tried with 
the 30th ; very faint traces, amounting to a mere suspicion 
of lines, was all it would exhibit ; possibly superior modes of 
illumination might produce better efi'ects with these glasses. 
The low powers, however, define objects suited to them very 
fairly. 
There are also grave mechanical defects in this instru- 
ment, which must be obvious to all. The supporting a long 
bar, carrying a considerable weight above, upon its lower 
extremity only, must of necessity produce unsteadiness ; and 
the sole adjustment being by rack and pinion, is so coarse that 
it is only with difficulty the higher powers can be brought 
into focus ; indeed, I have not succeeded in focussing the /-^th 
at all ; with these high powers the unsteadiness of the instru- 
ment is painfully apparent. With aU these disadvantages 
