REPORT OF THE SECRETARY. 65 



This device jiermits of indefinite extension of the bolometer arm, and consequently 

 the reduction of the angular vahie of the bolometer strip to a very minute quantity. 



(2) The provision of a new adjustable tangent arm for slowly rotating the axis 

 of the spectro-bolometer with great accuracy. 



(3) The adoption of a new system of clock-work for synchronously driving this 

 tangent arm, and the photographic plate on which the galvanometer record is 

 taken, in place of the two independent driving clocks before used. 



(4) The mounting of all parts of the spectro-bolometer on rigid iron or stoue 

 supports. 



The improvements in the method of working which have accompanied these 

 improvements in apparatus are : 



(1) The substitiitiou of glass plates for flexible films for the photographic records. 

 The irregular errors due to shrinkage of the film have thus been eliminated and the 

 subsequent iihotographic processes rendered much easier by reason of the greater 

 facility of handling. 



(2) The reduction, and in some cases almost entire elimination, of the "drift" by 

 the use of a water-jacket about the fixed bolometer case, together with cai'eful 

 attention to all the electrical details of the bolometer and galvanometer connections, 

 and the substitution of storage battery cells for the Daniel cells, formerly used to 

 supply the current to the bridge circuit. The " drift," however, still remains a 

 source of great trouble and I expect to secure its elimination only (if at all) by the 

 establishment of uniform temperature conditions, which it is impossible to obtain in 

 the present laboratory (at least during the summer months). 



The laboratory building itself has been considerably improved during the past 

 year. A small annex, which is used as a photographic dark room, was erected in 

 the spring of 1893, and has greatly facilitated the photographic work of the obser- 

 vatory. During the summer a small air-cooling plant was placed in the basement, 

 and served not only to increase the comfort of the observers, bat also to secure 

 more favorable conditions for the work then being carried on with rock salt. 



During the past year the observatory also fitted up a small Instrument shop for 

 the construction and repair of its apparatus, comprising an instrument-maker's 

 lathe, built to my special order, a small planer from the Hendley Machine Company, 

 and a fairly complete stock of small tools, and stock material. A dynamo, for sup- 

 plying current to the observatory for charging the storage batteries and to the shop 

 for power purposes, was also purchased and temporarily placed in the National 

 Museum. Owing to the lack of suitable quarters the shoi^ has not yet been perma- 

 nently located, but occupies a temporary shed south of the Smithsonian building. 



The important pieces of apparatus acquired during the year may be divided into 

 two general classes : (1) Physical apparatus of precision; (2) accessory apparatus. 



I. To the former class belongs : 



(1) Three new galvanometers and sets of galvanometer coils from Elliott 



Brothers and Nalder Brothers. 



(2) Resistance boxes one of 100,000 ohms and one of 1,000 ohms from Nalder 



Brothers. 



(3) A set of fine quartz fibers, from Prof. C. C. Hutchins. 



(4) Six fine galvanometer mirrors, from J. A. Brashear. 



(5) One large glass jirism, from J. A. Brashear. 



(6) Two large glass lenses, from J. A. Brashear. 



(7) Two new large rock salt lenses, from M. E. Kahler. 



(8) A collection of valuable rock salt crystals, from Germany. 



(9) Three new vVmilometer bolometers, from Grunow. 



(10) One large 24-inch camera, a fine Ross lens, and a complete photographic 



outfit, from Scoville »&. Co. 



(11) A new tangent arm for spectro-bolometer, from .J. A. Brashear. 



(12) A new prism holder, with large glass flat, from J. A. Brashear. 

 SM 93 5 



