﻿92 S. P. Langley — Unrecognized Wave-lengths. 



single vibration in 20 seconds a deflection of l mm division 

 of the scale is given by a current of 0*000,000,000,5 Amperes 

 through the coils of 20 ohms resistance ; and, as we have 

 just remarked, a tenth of one of these divisions can be dis- 

 criminated. 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 accordant measure- 

 ments have been made when the maximum deflection 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 l'6 mm 

 and most deflections less than one millimeter. On the other 

 hand, the exposure of the same bolometer to ordinary direct 

 sunlight with only 10 T 0o of the current passing, i. e. with the 

 galvanometer shunted 1,000 times, would drive the needle 

 immediately, and with violence, off the scale. 



Our experience has shown us that this galvanometer, in con- 

 junction with such a bolometer as we have described, is capable 

 of recording a disturbance of rather less than ioo0o 1 0000o part. 

 To attain corresponding accuracy in gravity determinations we 

 should need to have a balance capable of weighing a kilogram 

 which would give at the same time an unequivocal deflection 

 for a difference of one one-thousandth of a milligram in 

 either pan. A deflection of l mm 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 Trnhnr o °f a degree Centigrade, 

 and we have just seen that about J-g- of this can be shown. In 

 other words, about one- millionth of a degree can be indicated 

 by it, and a quantity less than one hundred thousandth of a 

 degree not only indicated but measured. It will be obvious to 

 the practiced observer, that this degree of accuracy will not be 

 in reality reached, unless the bolometer strips are perfectly pro- 

 tected from all extraneous radiations and air currents, and 

 especially unless the image is fixed upon the scale when the 

 bolometer is not exposed to heat. This degree of precision we 

 believe ourselves.to have actually obtained. 



Gratings. 



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 Professor Eowland, who 

 has been good enough to execute them for us of the very short 

 focus and open ruling, necessary for our particular work. 



Let us designate them as gratings No. 1, No. 2 and No. 3. 

 The dimensions of grating No. 1 have been given in a prior 

 memoir, but we repeat them here with some other data, for the 



