32 REPORT OF THE SECRETARY. 



a degree of accuracy only inferior to tliat whicli can be attained by the 

 eye. 



The instrument by which this is accomplished is the bolometer, which, 

 as now constructed, is a minute strip of metal barely -^y^ of an inch 

 wide, and less than 50-0 o of ^'^ i^^^i thick. Through this frail thread 

 of metal a current of electricity is continually kept flowing-. When 

 the spectrum, visible or invisible, is thrown upon it, the thread is 

 warmed and the current decreased by an amount corresponding to 

 the intensity of the effect received, while novel instruments specially 

 mounted and constructed, are in electric connection with the thread aud 

 now automatically record every minute change in this current. 



With late improvements these instruments are so delicate that a 

 change of temperature of one millionth of a degree is readily detected 

 nd even measured, and it is easy to see that as a consequence of this 

 delicacy the greatest care must be taken in their use. Thus the labora- 

 tory must be almost completely darkened, and closed tightly, so as to 

 exclude all drafts and to keep it at as nearly a uniform temperature 

 as possible, while for other reasons it must be kept under constant 

 hygrometric conditions. 



The passage of wagons or carriages in the neighboring streets even 

 is liable to cause serious trouble. Hence, the necessity for as complete 

 isolation of the laboratory as possible, aud the rigorous exclusion at 

 all times of all whose presence is not indispensable. 



In addition to these difficulties there are others of specially trying 

 character due to the nature of the work. 



To maintain other necessary conditions, the opening of a door or 

 window for purposes of ventilation is forbidden, even in summer, 

 although the temperature sometimes rises to 100° and even 110°, ren- 

 dering the work of observing in the small, non- ventilated and darkened 

 room very trying to the health of the observer. Frequent changes in 

 the staff" of the Observatory have been necessary for this and other 

 reasons, and the progress of the work has been delayed in consequence. 

 In spite of these and other difficulties, most of which are due to the 

 very temporary and inefficient nature of the small wooden building 

 in which the work is carried on, and its proximity to the traffic-laden 

 streets, the expectations of last year have been largely realized, and 

 a detailed publication of the work accompanied by charts showing 

 several hundred new and before unknown lines, will shortly be issued. 



Important as these results are, they are but the beginning of what 

 it may be hoped will be accomj)lished, with proper facilities. 



In view of what has been already accomplished, I hope that Con- 

 gress will see fit to make provision for the needs of the Astro-phys- 

 ical Observatory in the provision of a suitable site, the money for 

 a small permanent building being already available through the pro- 

 visions made by friends of the Institution whose contributions for this 

 purpose have already been acknowledged. 



