Prof. S. P. Langley on the Sew Sped nun. 12o, 



accuracy, as well as of delicacy, which would liaye appeared 

 impossible to the inventor himself in its early days. 



It may be considered in several relations, but notably as 

 to three : (1) Its sensitiveness to small amounts of heat ; 

 (2) the accuracy of measurement of those small amounts ; 

 and (3) the accuracy of its measurements of the position of 

 the source of heat. 



As to the first, it is well known that Ihe principle of the 

 instrument depends on the forming of a Wheatstone bridge, 

 by the means of two strips of platinum or other metal, of 

 narrow width and still more limited thickness, one of which 

 only is exposed to the radiation. In some bolometers in use, 

 for instance, the strip is a tenth of a millimetre, or one two- 

 hundred-and-fiftieth of an inch in width ; and vet it is to be 

 described as only a kind of tape, since its thickness is less 

 than a tenth of this. 



The use of the instrument is then based on the well-known 

 fact that the heating of an ordinary metallic conductor in- 

 creases its resistance, and this law is found to hold good in 

 quantities so small that they approach the physically infini- 

 tesimal. In the actual bolometers, for instance, the two arms 

 of a Wheatstone bridge are formed of two strips of platinum, 

 side by side, one of which is exposed to the heat and the other 

 sheltered. The warming of the exposed one increases its 

 resistance and causes a deflexion of the galvanometer. 



It was considered to be remarkable twenty years ago that 

 a change of temperature of one ten-thousandth of a degree 

 centigrade could be registered ; it is believed at present that 

 with the consecutive improvements of the original instrument 

 and others, including those which Mr. Abbot, of the Smith- 

 sonian Institution Observatory, has lately introduced into its 

 attendant galvanometer, less than one one-hundred-millionth 

 of a degree in the change of temperature of the strip can lie 

 registered. This indicates the sensitiveness of the instrument 

 to heat. 



As to the second relation, some measures have been made 

 on the steadiest light source obtainable. With ordinary 

 photometric measures of its intensity one might expect a 

 probable error of about 1 per cent. The error with the bolo- 

 meter was insensible by any means that could be applied to 

 test it. It is at any rate less than two one-hundredths of 

 1 per cent. If we imagine an absolutely invisible spectrum, 

 in which there nevertheless are interruptions of energy similar 

 to those which the eye shows us in the visible, then the bolo- 

 meter, whose sensitive strip passes over a dark line in the 

 spectrum, visible or invisible (since what is darkness to the 



