ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
697 
to use hydrogen for the mixed jet direct from the main, i. e. from the 
nearest gas bracket, so that only one bottle is necessary, and with 
proper management such a light is very economical, though, of course, 
it cannot compare with the paraffin light in this respect. 
The electric light, both arc and incandescent, has been extensively 
tried, but with disappointing results, and Mr. Kent has practically 
given it up, except in cases where it is necessary to take instantaneous 
photographs of moving objects. Even under such circumstances, direct 
sunlight, controlled by a suitable heliostat, is quite as efficient if the 
weather happens to be favourable. 
Whatever illuminant is ultimately chosen, a great deal depends on 
the condenser that is used. Rack and pinion for focusing the condenser 
is necessary, as also is some form of centering adjustment for placing 
the condenser accurately in the optic axis of the Microscope. Ear too 
little attention has hitherto been given to the advantages resulting from 
an intelligent use of the condenser, and it is not at all unusual to see, 
even in first-class laboratories, microscopists who keep their condenser 
invariably in one position and never think of focusing it accurately upon 
the object under examination. 
Very often, too, the stops or diaphragms are most unintelligently 
managed, and the result is an unsatisfactory visual image ; and if such 
an image is photographed, it must result in an unsatisfactory photograph, 
for one of the first things to be thoroughly understood, and one of the 
most important lessons that a photomicrographer can learn, is that the 
best visual image gives the photograph and no satisfactory negative 
can be hoped for unless a satisfactory visual image has first been 
obtained. 
Having then made certain that the image is accurately focused, 
another difficulty arises, viz. that of the exposure. And for exposure in 
photomicrography, just as for exposure in landscape work, no definite 
rules can be given. Practice only can teach the correct amount to be 
given to each preparation under different conditions of magnification 
and illumination. It is far better to use up a few plates at the 
beginning by making experiments than to attempt to be economical and 
learn exposure from books. 
Fuller, R. M. — An Improved Method of Photomicrography of Bacteria and other 
Micro-organisms. Med. Record, 1892, pp. 698-9. 
(5) Microscopical Optics and Manipulation. 
New Method for the Determination of the Refractive Indices 
of Anisotropic Microscopic Objects.* — Herr H. Ambronn gives a new 
method for determining the refractive indices of anisotropic microscopic 
objects. It consists in finding two liquids with refractive indices inter- 
mediate between the two refractive indices of the mineral section under 
examination. When immersed in such liquids and observed in polarized 
light, the object does not lose its outline when the axes of elasticity are 
parallel to the plane of polarization ; but between these positions an 
azimuth may be found in which a complete disappearance of outline 
does take place. This azimuth is determined in the case of the two 
* Ber. Verhandl. K. Sachs. Gesell. zu Leipzig, iii. (1893) pp. 316-8. 
