ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
591 
for even a series of experiments if long exposure is not required or 
more than weak currents. 
For long continuing investigations, especially when absolute exact- 
ness is desired, Flfisclrl’s brush electrodes may be advantageously used. 
A narrow glass tube 4 or 5 cm. long is cemented at its lower end with 
plastic clay, through which 
passes a fine short brush (fig. 
108, a and b) and the tube is 
three parts full of zinc sul- 
phate solution. The upper 
end is closed with a cork, 
through which passes a strong 
zinc wire, so that its lower 
end dips 1 cm. into the solu- 
tion. The part of the wire 
within the tube must be amal- 
gamated. The other end of 
the wire is bent and connected 
with the binding screw. The 
whole arrangement must be so 
adjusted as to lie exactly central with the groove previously strewn with 
kaolin, and the kaolin is also slightly sprinkled into the central hollow 
of the object-holder. The brush point thus lies on the kaolin mass 
which now forms the unpolarisable poles. When used with the trough, 
the brush is fitted to the insertions in the side walls. 
(4) Photomicrography. 
Photomicrography.* — Mr. Edmund J. Spitta furnishes a series of 
papers on this subject, which he treats under three heads (1) Low- 
power work ; (2) Medium-power work ; (3) High-power work. 
(1) Low-power worlc . — After pointing out that in this branch the 
Microscope is not used, he gives a useful series of practical directions. 
In discussing the choice of a lens he recommends one of short focal 
length. Where the object is large and the magnification required small, 
a diminutive portrait lens of about 3 in. focus, built on the Petzval prin- 
ciple and placed with its shorter conjugate focus towards the object, has 
given good results ; but if the magnification exceeds 1 J to 2 diameters, it 
is difficult for the operator to reach the milled head. However, Messrs. 
Dallmeyer have specially constructed a small rectilinear lens of about 
If in. focus, which, with magnifications over 1J to 2 diameters, has given 
really admirable results ; its only fault is that it is not constructed to 
work equally well with red, green, and violet rays. Mr. Spitta has 
found the finest type of lens to be the planar recently introduced by 
Zeiss, and constructed from calculations by Dr. Rudolph for the especial 
purpose of photomicrography. It works equally well on all colours of 
the spectrum, gives exquisite definition up to the margin of the plate, 
and is manufactured in several focal lengths. Its expensiveness is a 
matter for regret. An auxiliary front to the camera, attached where the 
lens usually fits, gives an increased extension of 6 or 8 in., and still 
* Pharm. Journal for 1898, pp. 326-9, 432-3, 474-5, 566-8, 587-9 (16 figs.). 
Fig. 108. 
