38 
Dr. Maddox, on Photomicrography . 
usual metliod of observation. To obtain this second or com- 
bining image, preference is given to every slight alteration in 
the illumination and focus, if not also in shifting the object 
a very little to the right or left. In these match-photo- 
graphs the images have to be kept down to the usual size of 
the stereo-plate, that the entire object may be combined ; or 
they may require to be overlapped. Some of the discs or 
more solid-shaped Diatomaceae, when copied as transparent 
positives and viewed in the stereoscope, have a charming 
appearance. 
The Actinophsenia, Coscinodiscus, and Craspedodiscus, 
from the Avonderful sculpturing on their surface when thus 
seen, keenly contest for the highest feelings of admiration, 
whether towards themselves as objects of innate beauty or 
towards the Great Artificer. The Diatomacese and many of 
the objects were mounted in balsam, and others in glycerine 
or glycerine and gum ; when the former were tried " dry,^^ 
the focussing in sunlight ay as uncertain. A few objects, 
from their colour, gave me considerable difficulty, some por- 
tions being quickly impressed, the other parts being Avithout 
any definition. The method by which I endeavoured to 
meet this Avas to folloAv up a suggestion of Mr. Shadbolt, 
namely, to give a somcAvhat similar colour to the whole object 
as had the densest portions ; this would naturally place the 
least coloured on a nearer equality with the most coloured as 
regards the actinic rays. The question Avas how to eflPect this 
without procuring a too lengthened exposure, as no form of 
heliostat was employed. First, I tried tinted glass beneath 
•the object, by which some advantage was obtained, yet not 
sufficient ; secondly, varnishing the back of the object-slide 
Avith a transparent, dark, resinous varnish was adopted ; this, 
although it answered to a very great extent, brought in a diffi- 
culty — the field of the negative had minature markings, which 
also pervaded the image. Therefore, setting aside both these 
methods, I determined on using a sIoav collodion and a rather 
acid bath. These measures enabled me to giA^e in some cases 
over seventeen seconds^ exposure in concentrated sunlight, 
of some objects, with excellent results, Avithout a trace of 
fogging or solarization, and the image was brought up to 
a normal density without much trouble. 
More than five years since, I tried bleaching certain dye- 
wood-cells with hydrochloric acid and chlorate of potash, after 
removing the colouring matter as much as possible by aid of 
solvents, but the cells were generally left Avith such an amount 
of roughness as to interfere with definition. After seeing 
that Dr. Hicks employed a similar plan to bleach insect tex- 
