166 PROGRESS OF MTCROSCOPICAL SCIENCE. [ jo^^SLl sl\T!? iff 
to make attempts, among whom may be mentioned Donne, Prof. Ger- 
lach, of Erlangen ; Joseph Albert, of Munich ; and Dr. E. L. Maddox, 
of Southampton, which proved that great usefulness could be claimed 
for the method. In America, Prof. O. N. Kood,* of Columbia College, 
Mr. Lewis M. Rutherfurd, of New York, and Dr. John Dean, of 
Boston, must be mentioned ; the latter having illustrated a paper on 
the spinal cord by photo-micrographs reproduced by photo-lithography, 
the magnifying powers not exceeding ten or twelve diameters, while 
both the former gentlemen had experimented with very high powers. 
Mr. Eutherfurd had published a paper upon Astronomical Photo- 
graphy,j" which contained many suggestions that the lecturer took 
advantage of ; and, after an interview with that gentleman, in which 
the plan of constructing the objective, the use of the ammonio-sulphate 
of copper, and the substitution of a properly constructed concave for 
the eye-piece, were discussed, he determined to develop these sugges- 
tions, and assigned to Assistant-Surgeon and Brevet-Major Edward 
Curtis, U.S.A., the duty of carrying out the manipulations ; to this 
gentleman he expressed his indebtedness for the successful issue of the 
experiments. The results attained were admitted by Dr. Maddox, 
who is one of the most successful labourers in this direction in Europe, 
and by other European savans, as most satisfactory, excelling any- 
thing heretofore done in this branch. To obtain successful photo- 
micrographs the following principles are involved : — 1. To use objec- 
tives so corrected as to bring the actinic ray to a focus. 2, To 
illuminate by direct sunlight passed through a solution of ammonio- 
sulphate of copper, which excludes practically all but the actinic 
extremity of the spectrum. 3. Where it is desired to increase the 
power of any objective, to use a properly constructed acromatic con- 
cave instead of an eye-piece. 4. To focus on plate-glass with a 
focussing glass, instead of on ground glass. 5. With high powers to 
use a heliostat to preserve steady illumination. 6. Where an object 
exhibits interference phenomena when illuminated with parallel 
rays, as is the case with certain diatoms and many of the soft 
tissues, to produce a proper diffusion of the rays by the interposition 
of one or more plates of ground glass in the illuminating pencil. 
The manipulations are made in a dark room, or rather one faintly 
illumined by yellow light ; the light used in the process of photograph- 
ing being admitted by a small brass tube, and is obtained from a 
mirror, which reflects the rays of the sun from a Silbermann's helio- 
stat, through the ammonio-sulphate cell before they enter the brass tube. 
When the plate of ground glass is used, it is placed between the mirror 
and the condenser, the diminution of light being overcome by a bull's- 
eye, where this may be necessary, as in the use of the higher powers. 
In order to enable those present to judge of the perfection of the work, 
he had the representations of some of the familiar test objects exhibited : 
first, there were thrown upon the screen, from photo-micrographic 
* " On the Practical Application of Photography to the Microscope." By 
Prof. O. N. Kood. Vol. xxxii., p. 186, ' Am. Journal of Science and Arts.' 
+ "Astronomical Photography." By Lewis M. Eutherfurd. 'Am. Jour. 
Sci. and Arts,' xxxix., 304, 
