ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
100 
For photographic purposes, only thin preparations which lie as much 
as possible in one plaue are suitable. For this reason diatoms have 
been the chosen subject of representation. Only the thinnest sections, 
as obtained by the most perfect microtomes, are suitable for photo- 
graphy. Of thicker preparations excellent drawings can be made. In 
not a few cases a certain thickness of the object is absolutely necessary, 
viz. when the essential details naturally lie in different planes. The 
draughtsman, by constant turning of the micrometer screw, combines the 
different planes into one ; in photography such a process is imprac- 
ticable. The discredit which has fallen upon photomicrography is for 
the most part to be ascribed to the ignorance and want of skill of 
those who would be always photographing objects which are absolutely 
unsuitable for photographic purposes. 
(5) Microscopical Optics and Manipulation. 
The Measurement of Lenses.* — The following is the valuable essay 
recently read by Prof. Silvanus P. Thompson to the Society of Arts : — 
Often have I regretted that the resources at the disposal of the Tech- 
nical College, Finsbury, did not enable its staff to organize and equip 
a proper laboratory for optical measurements, and for the standardizing 
of optical instruments, in the same thorough and practical way in which 
they have now for more than ten years organized laboratories for elec- 
trical measurements. What Prof. Ayrton did ten years back for the 
City and Guilds Institute, in organizing a laboratory for electrical 
measurements, I have longed to do for optical measurement, believing 
that, when the opportunity should come, the work would be of as 
much benefit to the optical industries of London as the electrical 
laboratories of the City and Guilds Institute have been. 
The exact measurement of optical quantities is no novelty: for in 
this branch of science precision has long reigned, if not in the lower 
branches of the industry, at least in the higher. And the laboratory 
methods of optical measurement are for the most part thoroughly 
worked out and known, though many of them unfortunately involve the 
use of expensive instruments and appliances. 
An optical laboratory should possess the means of testing rapidly, 
accurately, and without too expensive appliances, such matters as the 
truth of plane surfaces, the curvature of curved ones, the focal powers 
of lenses, their aberrations and their aperture. It should have means 
of testing mirrors and prisms, as well as actual entire instruments. It 
should be able to state the results in terms available for future years by 
the employment of accurate fundamental standards. 
With but one very small part of the subject of the work of the 
optical laboratory do I propose to deal to-night, namely, with the 
measurement of lenses. Lenses are used for many different purposes, 
and in varied functions and combinations. Measurements that would 
be important for some of these are utterly unimportant for others. For 
example, the condenser lenses used for magic lanterns are not wanted to 
be either aplanatic, achromatic, or rectilinear ; their function being 
merely to collect the light which emanates from a certain luminous 
* Journ. Soc. Arts, xl. (1891) pp. 22-39. Reprinted by the author’s permission, 
and with his corrections. 
