290 PROCEEDINGS OP THE AMERICAN ACADEMY 



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

 angular positions of the spectral lines 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 neces- 

 sary to substitute, for the observing telescope and micrometer eye- 

 piece 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 dif- 

 fraction 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 l'Ecole Normale, 2 serie, torn. iii. 

 p. 421 ; also in Journal de Physique, X., 1881, p. 425. It consists in 

 photographing images of the slit of the spectroscope upon the photo- 

 graphic plate, by turning the graduated circle of the spectrometer 

 through measured angles. These photographic images serve as fiducial 

 marks, by means of which wave-lengths of spectral lines on the plate 

 can be calculated. In the case of diffraction spectra obtained by de- 

 flecting a bundle of parallel rays at the angle of incidence, i, with a 

 deviation of order n, A n is connected to the wave-length A, and with a 

 certain constant, a, of the grating by the formula 



_ .An /. An\ 



2 a sin -~- cos \i 5- 1 = n A. 



It is evident that at least two errors can arise in the use of this formula ; 

 one from defective graduation of the circle of the spectrometer ; another 

 from the process of referring from the photographs of the slit on the 

 plate to the photographs of the metallic lines. 



We select the work of Hartley and Adeney * as perhaps the best 

 type of this method of using a camera with a spectrometer. Their 

 work is characterized by great care and thoroughness, and no one 

 could probably attain better results by the use of a flat grating, with 

 its concomitants of collimator, photographing lens, and camera. These 

 observers state that they were not troubled by the underlying spectrum 

 of a higher order than that which they photographed, for it was not 

 brought to a focus with the latter. In the new method we propose to 

 illustrate, all the spectra are in focus together, and this fact, instead of 



* Philosophical Transactions, CLXXV., 1884, pp. 63-137. 



