344 Messrs. Trowbridge and Sabine on Wave-lengths 



than in the infra-red region. If there are any harmonic rela- 

 tions between the wave-lengths of the spectra of metals, it is 

 here that one might expect to observe such relations. Indeed 

 Professor Griinwald, of Prague, has lately enunciated a 

 remarkable hypothesis upon the relations between the wave- 

 lengths of so-called elements, and finds in the observations of 

 various observers in the ultra-violet a strong confirmation of 

 his hypothesis. In any theoretical work upon the grouping 

 of spectral lines, it is of fundamental importance that the 

 wave-lengths of the lines should be determined with as great 

 accuracy as possible. The coincidence of metallic lines with 

 solar lines is at the best a doubtful piece of evidence. This 

 evidence is of moment only when the number of coincidences 

 becomes great, and is accompanied by characteristic grouping. 

 A mistake of a wave-length in the question of position is 

 sufficient to destroy the support which the author of any 

 hypothesis might claim for it. 



Conditions for Accuracy of Measurement. 

 All measurements of wave-lengths hitherto published have 

 been made by the old method of angular measurements with 

 a spectrometer. We say old, for the use of Rowland's con- 

 cave grating with its peculiar mounting must be characterized 

 as a new method and a new departure in measurements of 

 wave-lengths. The observation of wave-lengths of metallic 

 spectra by the eye is most laborious, and the photographic 

 plate must be substituted for the eye for most purposes. The 

 angular positions of the spectral fines on such a plate assume 

 great importance, for upon these positions depend the value of 

 the wave-lengths. In the operation of photographing spectral 

 lines, it is necessary to substitute for the observing-telescope 

 and micrometer-eyepiece of the spectrometer a camera-box 

 provided with a suitable lens, and with a plate-holder for the 

 photographic plate. Unless the latter is small the spectrum 

 will not be in focus on all parts of the plate ; moreover, unless 

 the distance of the photographic plate from the difiraction- 

 grating employed is comparatively large, the distances between 

 the spectral lines on the photograph will not be proportional 

 to wave-lengths. To determine these wave-lengths recourse 

 must be had to various devices. The one usually employed 

 is due to Cornu, and can be found described in the Annales de 

 VEcole Normale, 2 ser. tome iii. p. 421 ; also in the Journal 

 de Physique, x. 1881, p. 425. It consists in photographing 

 images of the slit of the spectroscope upon the photographic 

 plate bv turning the graduated circle of the spectrometer 

 througn measured angles. These photographic images serve 



