REPORT OF THE SECRETARY. G9 



(/) On the disperdon of rock salt. 



Tliiy subject has been investigated by several observers, first l)y yourself at Alle- 

 glieny about 1886, and since then by Julius, Paschen, Rubens and Snow, and others. 

 The object of making a further studj^ of it was to very materially increase the accu- 

 racy of determination in the infra-red portion of the spectrum, with the hope of 

 establishing the wave-lengths of the infra-red solar absorption lines discovered at this 

 Observatory to a degree of accuracy corresponding with the accuracy of our knowl- 

 edge of their mininnmi deviations in the spectrum of our great salt prism. There 

 appeared to 1)6 strong grounds for liope of succeeding in this endeavor, in the con- 

 sideration of the extraordinary facilities of the Observatory for such a research. 



Accordingly, the apparatus was made ready in July and August of 1898, and was 

 actually tried in the latter part of August. Active work was, however, deferred 

 until the latter part of December, and was done chiefly in January and February of 

 1899. While the method and results will be fully described in the forthcoming 

 publication, a brief statement will ])e appropriate here. 



The radiations of the sun were iised as the source of energy up to a wave-length 

 of 4//, but from this point to 6.5/^ (where the research was stopped partly because 

 the grating used was no longer applicable, and partly because further ]3rogress was of 

 no particular interest) the radiations of an iron gauze mantle heated in a Kitson 

 lamp' were employed. The radiations, from whichever source, fell first upon a slit 

 10 centimeters high, then upon a concave diffraction grating, and then upon the slit 

 of the spectro-bolometer, which remained practically as used in taking solar holo- 

 graphs. The grating apparatus being mounted according to Rowland's well-known 

 device, radiations whose wave-lengths were multiples of each other fell upon tlieslit 

 of the spectro-bolometer, and passed through to have their prismatic deviations 

 determined. For instance, if the apparatus was adjusted so that the well-marked 

 line of wave-length 0.5616// in the fourth-order spectrum was found by visual obser- 

 vation falling at the center of the slit, then it would be certain that radiations of 

 the following multiples of this wave-length also fell there: 4/3, 4/2, and 4/1. The 

 two latter would be well within the infra-red. 



Suppose, now, the driving clock of the spectro-bolographic apparatus to be started, 

 and a curve automatically produced just as a holograph would be. The form of this 

 curve would be a straight line only broken by the minute accidental deflections of 

 the galvanometer, except where the narrow bands of radiation at the above wave- 

 lengths caused narrow, steep-sided elevations. If we suppose, still further, that either 

 before or after this curve was made the direct sunlight was reflected upon the slit of 

 the spectro-ljolometer for several minutes, then a well-known portion of the solar 

 spectrum energy curve would appear in its proper relative position on the same plate, 

 and the positions of all the sharp elevations corresponding to known wave-lengths 

 could readily be measured on the comparator with reference to determined solar 

 absorption lines of the short holographs. Thus the wave-lengths at as many points 

 as desired could be determined without any circle readings whatever. 



Practically, this process (somewhat altered in details) was gone through with for 

 38 positions between wave lengths 0.76 /< and 6.5//; and not only once, but several 

 times, with all the care for accuracy which could be taken. As a result, it may be 

 said that the wave lengths of the al)sorption lines in the infra-red solar spectrum 

 discovered at this observatory can be told with an accuracy of about 3 i)arts in 

 10,000, while previous to this determination 1 part in 100 would have been all that 

 could be claimed. 



'This lamp, which burns vaporized petroleum oil, was very kindly placed by the 

 makers at the disposal of the Institution for the purpose. 



