164 Prof. 8. P. Langley on 



They have been taken under the following conditions: — 



(1) In the case even of the very lowest wave-lengths in 

 the feeblest heat we have been able to use a linear bolometer 

 of not more than 1 millim. width. 



(2) About an equal number of observations were intended 

 to have been taken with the prism placed so as to throw the 

 spectrum east and west. In doing this a minute systematic 

 error, amounting, at the greatest, to less than l' of arc, was 

 found to be caused by flexure of the arm, due to the weight 

 of the bolometer cable, and a correction for this has been 

 applied. Otherwise the observations are given as originally 

 made ; and as the " probable error ' 9 includes all the more 

 or less systematic differences, due to the use of different 

 gratings and different positions of the apparatus, it may be 

 considered to be in this case a fair indication of the amount 

 of error to be actually expected. 



We do not know of any determination of the change pro- 

 duced in the refractive power of a rock-salt prism by varying 

 temperature. A rough comparison of the deviations of 

 Fraunhofer lines, incidentally measured in the progress of 

 the work at different seasons, during which the temperature 

 has varied nearly 30° C, together with the results of a single 

 day's measures at temperatures differing by 17° C, have con- 

 curred in indicating a diminution in the deviations through- 

 out the visible spectrum of about 11" for a rise of temperature 

 of 1° 0. 



We do not doubt that a temperature-correction is also 

 required for the invisible spectrum ; but not having yet been 

 able to satisfactorily determine any, we think it best to leave 

 all the observations as they stand, uncorrected for tempera- 

 ture, and offer them under this reserve now, with the inten- 

 tion of returning to them hereafter. 



In the following Table we repeat, for convenience, the 

 results of certain optical measures made on September 14th, 

 1885, by Mr. J. E. Keeler with the rock-salt prism, whose 

 refracting-angle was on that date 59° 57' 54", and which we 

 have reduced to 60° by the formula given in this Journal 

 for May 1886. The wave-lengths are those due to Peirce's 



and Rowland's corrections of Angstrom, with which we have 

 been obliged by the authors before their formal publication. 



In the succeeding portion of the Table we have the re- 

 sults of measures made with the bolometer in the invisible 

 spectrum. The first column gives the source of heat; the 

 second the wave-length selected for measurement ; the third 

 the grating employed ; the fourth the refracting-angle of the 

 prism on the day of observation ; the fifth the temperature 



