64 REPORT — 1860. 



and that a Newtonian reflector, sustained on hollow trunnions, through one of 

 which the rays from the small mirror could come, would permit of operations being 

 carried on upon a horizontal table at the end of the trunnion with great ease. What- 

 ever might be the altitude or position of the object the photographic table would 

 always be horizontal. 



As I proposed that the telescope should not be less than 12 feet in focal length, 

 an advantage would obviously arise from making the vertical axis of the framework 

 beneath its centre of gravity. The observatory in which it should be placed would 

 then require to be only one-half the diameter that would otherwise be demanded. 

 A 12-foot tube could be worked with its frame in a cylindrical space, 13 feet in dia- 

 meter and 13 feet in height. 



I therefore cast a speculum of 15 inches in diameter and 2 inches in thickness. 

 The materials employed were Minnesota copper, regarded in America as the purest 

 commercial form of that metal, and Banca tin. Their proportions were those re- 

 commended by Lord Rosse. The cast was made in sand, 4 inches in thickness in 

 every direction from the speculum, which was permitted to remain for two days un- 

 opened, to ensure slow cooling. It proved to be perfectly successful. The machine 

 used for grinding and polishing it was that of Lord Rosse. 



The tube of the telescope is of black walnut, bound externally by brass rings, and 

 strengthened interiorly by iron ones. The trunnions at the little mirror are of gm*- 

 metal : they work on friction rollers of the same substance, supported on polished 

 steel axles. 



The telescope is moved in altitude, with the utmost facility, by the aid of counter- 

 poising levers, which act perfectly, whatever the position of the tube may be. The 

 pulleys through which these counterpoising levers work are also of gun-metal, sup- 

 ported on friction rollers, with polished steel axles. The motion, upon a vertical 

 axis, is accomplished by a cast-iron shaft, 2 t feet in length and 3 inches in thick- 

 ness, working at one end on a hemispherical termination in gun-metal, and the other 

 sustained in a strong and ground cast-iron collar. 



The observatory in which this instrument is being placed is situated on a hill, 

 400 feet above the level of the sea, at Hastings, about twenty miles north of New 

 York. 



The edifice consists of a sunken chamber, excavated out of the solid rock. The 

 walls of this chamber are substantially built of stone, laid in hydraulic cement. 

 Thev are 9 feet high. On the top of these walls a lighter wooden edifice is raised, 

 sufficient to make the building of the required height. The revolving roof is metallic. 

 The ground plan is square, and 17 feet in the clear interiorly. As the frame of the 

 telescope only requires a cylindrical space of 13 feet, the corners of the building are 

 very available for the necessary photographic preparations. 



On the top of the stone wall is placed a circular gallery running entirely around 

 the interior of the room, and enabling the operator to have access with great facility 

 to the photographic table and the eyepiece trunnion of the instrument. The in- 

 terior of the observatory is sheathed throughout with wood. 



This partly underground construction has been adopted for the purpose of ensu- 

 ring a more complete invariability of the temperature of the mirror. A thorough 

 ventilation is, however, secured whenever desirable, the local position of the edifice 

 being such that the door of entrance is on the side of the hill at the level of the floor. 

 The wooden sheathing is for the purpose of avoiding deposition of moisture. 



At the moment of writing this paper the building is unfinished, though rapidly 

 approaching completion. The various parts of the instrument and the photographic 

 arrangements are provided, and no difficulty is anticipated. 



This is the first observatory that has been erected in America expressly for celes- 

 tial photography, and it is hoped that, considering the purity of the skies, it will 

 yield good results. 



I expect also to derive considerable advantage from the method of darkening col- 

 lodion negatives by the aid of protochloride of palladium, described by me in a paper 

 read before the American Photographical Society, and which I think in this applica- 

 tion will permit of good proofs being taken by unprecedentedly short exposures. 



