1885.] Absorption-spectra Thermograms. 



17 



being made for temperature, the determination of the currents by this 

 method was abundantly accurate and very simple. 



The results are grouped in three series, of which the first two were 

 considered in the Preliminary Note. In both of them the same tube 

 was used, the principal difference being that in the first the light 

 traversed the tube three times, and in the second but once. In the 

 third series another tube was employed, and some improvements in 

 respect to thermal insulation were introduced. The readings were 

 taken with a double image prism in place of the ordinary analysing 

 nicol, a substitution by which it is believed some advantages were 

 obtained. 



From the fifteen sets of observations of Series I, we find as the 

 rotation of sodium light in bisulphide of carbon at 18° corresponding 

 to a difference of potential equal to unity C.G.S. the value '04203 

 minute. From the four observations of Series II we get in like 

 manner "04198 minute, and from the seven observations of Series III 

 "04202 minute. The last value is adopted as the most probable. 



In an appendix some remarks are made upon polarimetry in general, 

 especially in relation to the half -shade method. A device proposed by 

 M. Becquerel for augmenting the precision with which rotations can 

 be determined with the aid of a half -wave plate is considered, and the 

 conclusion is arrived at that no advantage can thus be obtained. 



III. " Absorption-spectra Thermograms." By Captain Abney, 

 R.E., F.R.S., and Colonel Festing, R.E. Received 

 December 31, 1884. 



When employing a grating for visual work, the choice of a medium 

 which shall absorb the overlapping parts of orders of the spectrum 

 other than that under examination is a comparatively simple affair. 

 When working photographically, however, it becomes necessary to 

 know what invisible rays the medium will cut off. For instance, in 

 investigations in the infra-red of the spectrum, we have found it 

 necessary to ascertain whether media which absorb the blue and allow 

 the red rays to pass would also transmit the above-mentioned parts of 

 the spectrum. 



Photography would seem to be the simplest method of experiment- 

 ing in this direction ; but the results not being quantitative, as are 

 those obtained by a thermopile, we found it better to work with that 

 instrument. 



It should be remembered that we were dealing with the infra-red 

 part of the spectrum, in which the energy is so great as to be well 

 shown by the thermopile ; in the more refrangible part the indications 



