ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
657 
The worker in photomicroscopy, who uses plates having opposite 
qualities as regards density, contrast, and range of tones, and who uses 
them intelligently, will obtain results which cannot be equalled by the 
one who uses one make of plates only, or who uses all kinds as may 
happen, without a knowledge of their properties arrived at by compara- 
tive testing.” 
Marktanner-Turneretscher’s ‘Die Mikrophotographie als Hilfs- 
mittel Naturwissenschaftlicher Forschung.’* — The aim of this little 
work on photomicrography is to afford assistance to those who wish to 
make use of photomicrography in their investigations, so that they may 
attain their object with as little expenditure of time and trouble as 
possible. The theoretical considerations are supplemented by a number 
of very serviceable practical hints. After a brief sketch of the history 
of photomicrography and its uses, the author gives a description of a 
complete photomicrographic apparatus, and explains the various uses and 
modes of production of the different sources of light. He then deals 
with the properties of photomicrographic preparations and gives a 
concise but comprehensive account of the practical operations which are 
necessary for the production of a good photomicrogram. The usefulness 
of the book is increased by numerous bibliographical references, good 
illustrations, and well-executed photomicrograms. 
(5) Microscopical Optics and Manipulation. 
Diatom-Structure— The Interpretation of Microscopical Images.f — 
Dr. J. D. Cox in speaking on this subject made the following among 
other remarks : — 
“ In such a case the real question is one of interpretation of ap- 
pearances seen under the Microscope, and what I have to say will 
bear chiefly on this point, with direct application to the study of 
diatoms. 
All microscopists are acquainted with the position of Prof. Abbe in 
regard to images formed by diffraction. As commonly stated it amounts 
to a declaration that all microscopical images of structure with details 
smaller than -0005 of an inch are diffraction images from which the 
true structure may be argued, but which cannot be taken as in them- 
selves true representations of the structure. 4 The resulting image 
produced by means of a broad illuminating beam,’ says Prof. Abbe,! 
‘ is always a mixture of a multitude of partial images, which are more 
or less different (and dissimilar to the object itself).’ 
This theory has been very vigorously assailed by Mr. E. M. Nelson, 
of London, from the practical and experimental side. In a paper read 
before the Quekett Club in May [1890], entitled “ The Substage Con- 
denser : its history, construction, and management ; and its effects 
theoretically considered,” Mr. Nelson asserts that the cone of light 
from a substage condenser 4 should be of such a size as to fill 3/4 of the 
back of the objective with light ; thus N.A. 1*0 is a suitable illumi- 
nating cone for an objective of 14 N.A.’ He says that * this opinion 
* Biol. Centralbl., xi. (1891) p. 351. 
f Journ. New York Micr. Soc., vii. (1891) pp. 76-87. 
X R. M. S. Journal, December 1889. 
