S. P. Langleij — Optical Properties of Rook-salt. 479 



To make all observations strictly comparable, they are re- 

 duced to one value of the refracting angle, for which the devia- 

 tions of the Fraunhofer lines and the wave-lengths corres- 

 ponding to given deviations in the infra-red, tutve been deter- 

 mined with the greatest possible accuracy. This standard value 

 of the refracting angle is 59° 57' 54". A series of observations 

 for fixing the positions of the Fraunhofer lines was made by 

 Mr. J. E. Keeler of this observatory, on September 14th, . 18S5. 

 One arm of the spectrometer, which was firmly clamped, carried 

 a glass collimating lens of 25 feet focus, and the other an 

 achromatic observing telescope of nearly four feet focus, with 

 a mycrometer eyepiece. The double deviations of the C, D 15 b 1? 

 and F lines were observed, and also the differences of deviation 

 between these and the other lines whose positions were deter- 

 mined. For observing the M and N lines a Soret fluorescent 

 ej^epiece was used, and in the infra-red a bolometer, having a 

 single strip -fa millimeter in width. In the two last cases the 

 prism was automatically kept in the position for minimum de- 

 viation. The spectrometer circle reads by two opposite ver- 

 niers to 10", but on account of the construction of the instru- 

 ment, (for whose principal purpose arms whose length is incon- 

 sistent with absolute rigidity had to be used) care is necessary 

 to measure an angle with this degree of precision, as the arms 

 are liable to spring slightly on the application of lateral pres- 

 sure. The deviations given in our table were obtained by Mr. 

 Keeler by setting om the line, with the micrometer eyepiece, 

 after the telescope had been directed upon it and freed from 

 strain by a light tap, and applying the micrometer correction 

 to the circle reading. It was found by careful comparison of 

 the solar spectrum given by the rock-salt prism with that by a 

 fine prism of flint glass, that in spite of the greater dispersion 

 of the latter, no lines could be seen in its spectrum which the 

 rock-salt prism would not also show. The probable error of 

 one setting of the micrometer was less than 1". From the 

 agreement of the different measurements made in this way, it 

 is believed that the deviations throughout the visible spectrum 

 are correct to within 5". Those in the ultra-violet and infra- 

 red, cannot of course pretend to this degree of accuracy. The 

 positions of par (invisible) given by two independent series 

 with the bolometer differed by 30"; those of <p (invisible) by V, 

 those of (p (invisible) by 30" and those of Q (invisible) agreed 

 exactly. 



We have thus obtained incidentally the data for constructing 

 a table of refractive indices of rock-salt throughout the entire 

 range of the solar spectrum, with an accuracy which we believe 

 to be greater than has heretofore been attained, and which we 

 deem of sufficient interest to give in full below, for the con- 



