158 Prof. S. P. Langley on 



For the purpose of forming an estimate of the sensitiveness 

 of the instrument it may be stated that, when making a single 

 vibration in 20 seconds, a deflection of 1 millim. division of 

 the scale is given by a current of 0-000,000,000,5 amperes ; 

 and, as we have just remarked, a tenth of one of these divi- 

 sions can be discriminated. That this degree of sensitiveness 

 is associated with a real, and not nominal corresponding degree 

 of accuracy, is shown by the fact that many series of accord- 

 ant measurements have been made when the maximum de- 

 flection did not exceed three such divisions ; and that similar 

 measures have been made in the invisible spectrum given by 

 ice melting in a dark room, when the maximum deflection 

 observed was 1*6 millim., and most deflections less than one 

 millimetre. On the other hand the exposure of the same 

 bolometer to ordinary direct sunlight with only jq'oo °f ^he 

 current passing, i. e. with the galvanometer shunted 1000 

 times, would drive the needle immediately, and with violence, 

 off the scale. 



Our experience has shown us that this galvanometer, in 

 conjunction with such a bolometer as we have described, is 

 capable of recording a disturbance of rather less than ^ooq^ooq 

 part. To attain corresponding accuracy in gravity determi- 

 nations we should need to have a balance capable of weighing a 

 kilogramme, which would give atthe same time an unequivocal 

 deflection for a difference of one one-thousandth of a milli- 

 gramme in either pan. A deflection of 1 millim. corresponds, 

 in the case of such a bolometer as we have used in the lunar 

 spectrum, or in that of melting ice, to a change of temperature 

 in the bolometer-strips considerably less than 100)000 of a degree 

 Centigrade, and we have just seen that about y 1 ^ of this can 

 be shown. In other words, about one one-millionth of a degree 

 can be indicated by it, and a quantity less than one one- 

 hundred-thousandth of a degree, not only indicated, but 

 measured. It will be obvious to the practised observer that 

 this degree of accuracy will not be in reality reached unless 

 the bolometer-strips are perfectly protected from all extra- 

 neous radiations and air- currents, and especially unless the 

 image is fixed upon the scale when the bolometer is not ex- 

 posed to heat. This degree of precision we believe ourselves 

 to have actually obtained. 



Gkatings. , 



Of the concave gratings we have three, of the very largest 

 size. These magnificent instruments we owe, not only to the 

 skill, but to the special kindness of Prof. Kowland, who has 



