A SILVERED GLASS TELESCOPE. 33 



large enough to contain the tube of the telescope. At right angles to the parallelo- 

 gram below, and close upon it, a braced bar o o', Fig. 28, crosses. From its ex- 

 tremities four slanting braces as at p p' , Fig. 28, go to the corners of the upper 

 parallelogram, and combine to give it lateral support. At the top of one close 

 pair of the perpendicxilars n' , Fig, 28, are bronze frames carrying friction roUers 

 upon which the trunnions move, whUe similarly upon the other pair n are two 

 pulleys, also on friction roUers, for the wire rope coming from the counterpoises. 



Movement in altitude is very easily accomplished, and with the left hand upon 

 the winch ^, under high powers, both altitude and azimuth motions are controlled, 

 and the right hand left free. The whole apparatus works so well, that in ordinary 

 observation the want of a clock movement has not been felt. Of course for pho- 

 tography that is essential. 



§ 3. THE CLOCK MOVEMENT. 



The apparatus for following celestial bodies is divided into two parts ; a. The 

 Sliding Plate-holder ; and b. The Clepsydra. In addition a short description of the 

 Sun-Camera, c, is necessary. 



a. The Sliding Plate-holder. 



Mr. De La Eue, who has done so much for celestial photography, was the first 

 to suggest photographing the moon on a sensitive plate, carried by a frame moving 

 in the apparent direction of her path. He never, however, applied an automatic 

 driving mechanism, but was eventually led to use a clock which caused the 

 whole telescope to revolve upon a polar axis, and thus compensate for the rotation 

 of the earth, and on certain occasions for the motion of the moon herself. In this 

 way he has produced the best results that have been obtained in Europe. Lord 

 Eosse, too, employed a similar sliding plate-holder, but provided with clock-work 

 to move it at an appropriate rate. I have not been able as yet to procure any 

 precise account of either of these instruments. 



The first photographic representations of the moon ever made, were taken by 

 my father, Professor John W. Draper, and a notice of them published in his quarto 

 work " On the Forces that Organize Plants," and also in the September number, 

 1840, of the. London, Edinburgh, and Dublin Philosophical Magazine. He pre- 

 sented the specimens to the New York Lyceum of Natural History. The Secretary 

 of that Association has sent me the following extract from their minutes : — 



" March 2Sd, 1840. Dr. Draper announced that he had succeeded in getting a 



representation of the moon's surface by the Daguerreotype The time 



occupied was 20 minutes, and the size of the figure about 1 inch in diameter. 

 Daguerre had attempted the same thing, but did not succeed. This is the first time 

 that anything like a distinct representation of the moon's surface has been obtained. 



" RoBT. H. Brownne, Secretary." 



As my father was at that time however much occupied with experiments on the 

 *^-hemical Action of Light, the Influence of Light on the Decomposition of Car- 



