S. P. Langley — Bolometer. 243 



Since then the bolometer has been used in various researches, 

 of which some occasional account has been given in this 

 Journal. (See numbers for .November, 1888, and August, 

 1890.) During recent years it has been specially employed in 

 making a holographic map of the lower spectrum, the publica- 

 tion of which has been greatly delayed by conditions incidental 

 to the relations of the Smithsonian Observatory with the 

 Government, but which it is hoped will not be deferred much 

 longer. 



Without here entering into an account of the work done by 

 it, I have thought that it might be of interest to give very 

 briefly a statement of the present condition and form of the 

 instrument itself, considered under three aspects : 



1. Its precision, or the degree of exactitude with which it 

 can be set on a special point, as, for instance, on a line of the 

 invisible spectrum, recognized by its heat radiation alone. 



2. Its accuracy, or its capacity for repeating the same meas- 

 ure of radiation under like conditions. 



3. Its sensitiveness, or capacity for detecting minute radia- 

 tions. 



The instrument which I will take as the subject of compari- 

 son with the earlier one as described in this Journal (August, 

 1886), is now in use in a chamber automatically kept at a tem- 

 perature, constant within one-tenth of a degree centigrade. 



The strips, the essential part of the instrument, are in the 

 present case made by Mr. C. G. Abbot, and are of platinum, 

 the central one being rather less than 0*1 of a millimeter wide. 

 (The case is now made of metal instead of ebonite, and is 

 surrounded by a current of water.) 



It is quite possible to make bolometer strips much narrower, 

 but this is less necessary with the employment of the long- 

 focus, image-forming mirror, so that in the present case the 

 strip is at such a distance that it subtends an angle of 3*4 sec- 

 onds. Its angular aperture is in practice adapted to that of 

 the slit, which, with the use of the long collimator employed 

 by the writer, gives a capacity of pointing (pointing, 

 that is, in the dark), with a probable error of little over a 

 second of arc. Quite recently, owing to the use of a novel 

 collimating system of two cylindrical mirrors proposed by Mr. 

 Abbot, the slit, though at a moderate distance, can have an 

 opening sufficient to avoid prejudicial diffraction effects, while 

 subtending an angle of considerably less than one second of arc. 



In the galvanometer, the use of the fine quartz threads and 

 specially small mirrors, originally due to Mr. Boys, has lately 

 been carried to what seems near the practicable extreme, the 

 quite invisible thread being made some 30 centimeters long, 

 the mirror 2 millimeters in diameter, and weighing but 2 milli- 



