and Unmeasured Wave-lengths, 407 



rapid according to the greater or smaller relative humidity of 

 the atmosphere at the time. In ordinary dry weather they 

 may be used several times before they become spoiled, while 

 in clamp or rainy weather three or four seconds is a sufficiently 

 long time to cover them with condensed moisture, and work 

 under these circumstances is of course impossible. After the 

 surfaces have in this way become unfit for use they are re- 

 polished, and the refracting angle of the prism is thereby 

 unavoidably altered. The change is usually small, generally 

 not exceeding V, so that for most of our heat-measures it may 

 be neglected altogether. The changes have, however, tended 

 on the whole to reduce the refracting-angle, so that it is now 

 about 4/ smaller than when the prism was first used. 



To make all observations strictly comparable, they are 

 reduced to one value of the refracting-angle, for which the 

 deviations of the Fraunhofer lines, and the wave-lengths 

 corresponding to given deviations in the infra-red, have been 

 determined with the greatest possible accuracy. This standard 

 value of the refracting-angle is 59° 57' 54 /x . A series of 

 observations for fixing the positions of the Fraunhofer lines 

 was made by Mr. J. E. Keeler, of this Observatory, on Sept. 

 14, 1885. 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 micrometer-eyepiece. The double deviations 

 of the C, D 1; b i? and F lines were observed, and also the 

 differences of deviation between these and the other lines 

 whose positions were determined. For observing the M and N 

 lines a Soret fluorescent eyepiece was used, and in the infra- 

 red a bolometer, having a single strip -^ millimetre in width. 

 In the two last cases the prism was automatically kept in the 

 position for minimum deviation. The spectrometer-circle 

 reads by two opposite verniers to 10", but on account of the 

 construction of the instrument (for whose principal purpose 

 arms whose length is inconsistent 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 pressure. The deviations given 

 in our table were obtained by Mr. Keeler by setting on 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 a 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 



