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. End of the Project Gutenberg EBook of On the Construction of a Silvered Glass Telescope, by Henry Draper