S. P. Langley — New Spectrum. 407 



it has grown to something very different from what it was 

 then. 



It has, in fact, since found very general acceptance among 

 physicists, especially since it has lately reached a degree of 

 accuracy, as well as of delicacy, which would have 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 the 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 millimeter, or one 

 two-hundred-and-fiftieth of an inch in width ; and yet 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 deflection of the galvanometer. 



It was considered to be remarkable twenty years ago that 

 the 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, that less than one one- hundred- mil- 

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

 be registered. This indicates the sensitiveness of the instru- 

 ment to heat. 



As to the second relation, some measures have been made 

 on the steadiest light source obtainable. With ordinary photo- 

 metric measures of its intensity one might expect a probable 

 error of about one per cent. The error with the bolometer 

 was insensible by any means that could be applied to test it. 

 It is at any rate less than 7 §„ of one per cent. If we imagine 

 an absolutely invisible spectrum, in which there nevertheless 

 are interruptions of energy similar to those which the eye shows 



