engaged in the invention of portrait photography, he found that the
ammonio-sulphate of copper, a deep blue liquid, will separate the more
refrangible rays of light, the rays concerned in photography, from the
rest. If a beam of sunlight be passed through such a solution, inclosed
between parallel plates of glass, and then condensed upon an object on
the stage of a microscope, a blue colored image will be formed on the
ground glass, above the eyepiece. If the place of best definition be
carefully ascertained, and a sensitive plate put in the stead of the
ground glass, a sharp photograph will always result.
Besides, there is no danger of burning up the object, as there would
be if the unabsorbed sunlight were condensed on it, and hence a much
larger beam of light and much higher powers can be used. The best
results are attained when an image of the sun produced by a short
focussed lens is made to fall upon and coincide with the transparent
object. In 1856 we obtained photographs of frog’s blood disks,
navicula angulata, and several other similar objects under a power of
700 diameters, excellently defined. Since then several hundreds of
microscopic pictures have been taken.
In the figure, _a_ is the heliostat, _b_ a lens of three inches
aperture, _c_ the glass cell for the ammonio-sulphate of copper, _d_
the object on the stage of the microscope _e_, _f_ the camera for the
ground glass or sensitive plate. Above the figure the course of the
rays is shown by dotted lines.
* * * * *
In concluding this account of a Silvered Glass Telescope I may answer
an inquiry which doubtless will be made by many of my readers, whether
this kind of reflector can ever rival in size and efficiency such
great metallic specula as those of Sir William Herschel, the Earl of
Rosse, and Mr. Lassell? My experience in the matter, strengthened by
the recent successful attempt of M. Foucault to figure such a surface
more than thirty inches in diameter, assures me that not only can the
four and six feet telescopes of those astronomers be equalled, but even
excelled. It is merely an affair of expense and patience. I hope that
the minute details I have given in this paper may lead some one to make
the effort.
HASTINGS, WESTCHESTER COUNTY,
NEW YORK, 1863.
_Postscript._--Since writing the above I have completed a photograph
of the moon 50 inches in diameter. The original negative from which it
has been made, bears this magnifying well, and the picture has a very
imposing effect.
PUBLISHED BY THE SMITHSONIAN INSTITUTION,
WASHINGTON CITY,
JULY, 1864.