46 



ON THE CONSTRUCTION AND USE OF 



_ts general arrangement is better comprehended from the sketch, Fig. 42, than 

 from a labored description. Below, it runs on a pair of wheels a (one only is 



Fig. 42. 



The Observer's Chair. 



visible) 9 inches in diameter, whose axles point to the centre of the circle upon 

 which they run. They are prevented from shifting outwards by a wooden railroad 

 b, b', and inwards by the paling Z, I'. Above, the chair moves on a pair of small 

 rollers c, which press against a circular strip or track d, d', nailed around the lower 

 edge of the dome opening. Access to the platform is gained by the steps e, e'. 

 Attached to the railing of this platform, and near it on the telescope, are two 

 tables (not shown in the figure) for eyepieces, the sliding plateholder, &c. 



5. THE PHOTOGRAPHIC LABORATORY. 



This section is divided into a, Description of the Apartment ; and b, Photographic 

 Processes. 



a. Description of the Apartment. 



The room in which the photographic al operations are carried on, adjoins and 

 connects with the observatory on the southeast, as is shown in Figs. 28 and 38. 

 It is 9 by 10 feet inside, and is supplied with shelves and tables running nearly all the 

 way round, which have upon them the principal chemical reagents. It is furnished, 

 too, with an opening to admit, from a heliostat outside, a solar beam of any size, up 

 to three inches in diameter. 



The supply of water is derived from rain falling on the roof of the building, and 



