A SILVERED G LASS TELESCOPE. 55 



concerned in photography, from the rest. If a b(>iuu of sniih'<rht be passed through 

 such a sohition, inclosed between parallel ])lates of glnss, and tlien condensed upon 

 an object on the stage of a microscope, a bhu> 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 ])hotograph 

 will always result. 



Besides, there is no danger of burning up the objettt, as tliere 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 wJien an image 

 of the sun produced by a short focussed lens is made to fall upon and coincide with 

 the transparent object. In lcS56 we obtained photographs of frog's blood disks, 

 navicula angulata, and several other similar objects uiuler 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 apertiire, 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 refiector 

 can ever rival in size and efficiency such great metallic speciUa as those of Sir 

 William Herschel, the Earl of Rosse, and Mr. Lasselll My experience in the 

 matter, strengthened ]»v 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 eftbrt. 



Hastings, Westchester County, 

 New York, 1863. 



Postscript . — Since writing the above I have completed a ])hotograph of the moon 

 50 inches in diameter. The original negative from which it has been maile, bears 

 this magnifying well, and the picture has a very imposing eflect. 



PUB1,I.SHEI) UV THE S M T T II SO N I .\ N INSTITUTION, 

 W A S 11 I N U T O N CITY, 



JULY, 18G4. 



