A SILVERED GLASS TELESCOPE. 65 



concerned in photography, from the rest. If a beam of sunlight be passed through 

 such a sohition, 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 ascer- 

 tained, and a sensitive plate put in the stead of the ground glass, a sharp photograph 

 wUl 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, 

 navicida 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, h 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, 

 / 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 wdl 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 

 WiUiam Herschel, the Earl of Rosse, and Mr. LasseU? 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 afi'air of expense and patience. I hope that the minute details I have 

 given in this paper may lead some one to make the eff'ort. 



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 efi"ect. 



PUBLISHED BY THE SMITHSONIAN INSTITUTION, 

 WASHINGTON CITY, 



JULY, 1864. 



