218 CORRESPONDENCE. 
objective that the observer may wish to employ. But neither plan is 
of any use for centring objectives to the achromatic condenser when it 
is eccentric; moreover, the above-adjustments are cumbersome, because 
they must be fixtures, and their use is limited. 
The “new centring nose-piece” is, in my opinion, a great boon 
to observers who work with high powers on ¢est objects. Test work 
may not be very instructive, but it is undoubtedly the cause of the 
great improvements made on some of the modern objectives; it is also 
fashionable, not only among amateurs, but even among savants; and 
therefore any new apparatus that facilitates the attainment of accuracy 
should be welcomed. 
The centring nose-piece is made by Mr. Swift, and it figures in 
‘Science-Gossip’ for February, page xv. The rectangular adjust- 
ments act in the same manner as those of the sub-stage, and the 
objective is thereby adjusted to the optical axis of the instrument. 
This nose-piece has the following advantages, namely: Ist, it is not a 
fixture, and it works with any microscope fitted with the “ universal 
screw”: 2nd, it may be used to rectify the eccentricity of the achro- 
matic condenser caused either by the “spring” of the fine adjustment 
when moving the screw-collar of the objective, or by the unsteadiness 
of the sub-stage; and also to centre any objective to the achromatic 
condenser fitted to a microscope without any under-stage adjustments: 
8rd, it may also be used to render the rotating stage concentric with 
any objective. The only drawback is that this nose-piece may slightly 
strain the fine adjustment. 
It must be taken into consideration that neither the centring of 
the stage, nor that of the objective, is obtained “instantaneously ” 
nor “with the greatest facility ”: in either case the adjustments must 
be made very carefully. 
I am, Sir, yours obediently, 
A. DE SouzA GUIMARAENS. 
Nore ON THE STRUCTURE oF THE TEest IN ARCELLA. 
To the Editor of the ‘ Monthly Microscopical Journal. 
Sir,—In the ‘ Quarterly Journal of Microscopical Science’ for 
January last, p. 79, the following statement appeared : 
“For the first time seemingly a correct description of the structure 
of the peculiar test of the somewhat variously and otherwise pretty 
well known and at least common species Arcella vulgaris (Ehr.), is 
given by Hertwig and Lesser. Two plates, an outer forming the 
superficies of the test, an inner applied to the body of Arcella, are 
united in a honeycomb-like structure, whose hexagonal cavities form 
prismatic spaces standing vertically on the surface. Hertwig and 
Lesser conclude that the appearance of the markings on the Arcella 
test is not due to granulation as Dujardin supposed, or Claparéde and 
Lackmann as well as Carter assumed. Wallich indeed spoke of 
symmetrical reticulation and of hexagonal interspaces ; still Hertwig 
and Lesser doubt if he altogether correctly appreciated the structure, 
