46 



ON THE COXSTRUCTION AND USE OF 



Its general arrangement is better comprehended from the sketch, Fig. 4"2. tlian 

 from a labored description. Below, it runs on a pair of wheels a (one oidy 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 pre^ented from shifting outwards by a wooden railroad 

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

 rollers c, which press against a circular strip or track (/, il', 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 tliis platform, aiul near it on tlie telescope, are two 

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



§ 5. THE PHOTOGRAPHIC LABORATORY. 



This section is divided into a. Description of the Apartment ; and 6, Piiotographic 

 Processes. 



a. Description of ihe Ajjartment. 



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

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

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

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

 too, with an opening to admit, from a lieliostat 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 



