43 8 The Smithsonian Institution 



show, on simple inspection, not only the inflection of the 

 energy curve there written down, but the exact relative posi- 

 tion in the distant spectrum which the bolometer thread oc- 

 cupied at the moment it caused the disturbance. By suitably 

 changing the wheels in the clockwork we may cause the 

 spectrum to move fast or slow, in the former case giving only 

 its principal inflections, in the latter case giving a great deal 

 more of detail, but with liabilities to error, which will be 

 spoken of later. 



The building shown in the annexed sketch plan, which 

 was erected in 1890 at the cost of the Institution, has been 

 slightly modified from its original form to meet the wants of 

 this process as they have been developed by experiment, and 

 it is only lately that the small photographic room shown at 

 the right has been added. The observatory's latitude and 

 longitude as given by the United States Coast and Geodetic 

 Survey is 38 53' I7".3 and 5 h o8 m o6 s .24 respectively. 



The building is essentially a room arranged so that it can 

 be closed to all light by means of sliding shutters before the 

 windows, and by a sliding shutter under the skylight in the 

 roof, and containing an inner chamber F F which can be kept 

 at a constant temperature. In the front is a small room L 

 containing books and writing materials, below which is a 

 cellar in which are stored the batteries and a furnace, the 

 latter being no longer used, having been replaced by steam 

 radiators R, operated from another building. Around the 

 walls are cases containing those pieces of apparatus which 

 are not in constant use, and, with the exception of the small 

 cellar, the floor joists are almost in contact with the soil, but 

 piers for the instruments rise to the level of the floor at A, 

 76 centimeters above the floor at B, and 40 centimeters 

 above the floor at J. When the shutters are closed the 

 only light which enters comes from the siderostat at C, which 



