INTRODUCTORY. 7 



PHOTOGRAPHIC MATERIAL, ETC. 



It was at first decided to use films coated with the panchromatic emul- 

 sion made by Wratten and Wainwright, of Croyden, England, as this emul- 

 sion has been found to be very uniformly sensitive to light of all wave- 

 lengths between A 2000 and about A 7400. The makers, however, did not 

 succeed in producing a satisfactory film in time for the present work, and 

 hence it was necessary to use the Seed L-Ortho-film for the region from 

 A 2000 to about A 6000, and to make a separate exposure for the red end of 

 the spectrum, using for this purpose Wratten and Wainwright panchro- 

 matic glass plates, cut, to such lengths (4 to 4.5 inches) that the curvature 

 of the focal plane would not introduce any appreciable difficulty. This 

 method very nearly doubled the time and work consumed in making the 

 spectrograms, as two separate sets of exposures had to be made with each 

 set of solutions. It also made it very difficult to get the two negatives of 

 such intensities that they would match satisfactorily, owing not only to 

 the different absolute sensibility of the two emulsions to light of a given 

 wave-length, but also to the different rates at which the photographic black- 

 ening increases with time for the two emulsions. 



Plate 1 gives some idea of the sensibility of the photographic plates 

 used to light of different wave-lengths, and also shows how the photo- 

 graphic action increases with time of exposure. A is a series of exposures 

 of the Wratten plate to the spectrum of the Nernst filament, the times of 

 exposure being 2, 4, 6, 8, 15, and 30 seconds, and 1 and 2 minutes, respec- 

 tively. 0.8 ampere of alternating current flowed through the filament 

 and the slit was adjusted to a width of 0.01 cm. The strip on the nega- 

 tive corresponding to the 2-seconds exposure shows some photographic 

 action from A 3800 to A 7250 with faint maxima near A 4700, A 5400, A 5950, 

 A 6500, and A 6950, respectively; the corresponding minima falling near 

 A 5050, A 5600, A 6100, and A 6700. The minimum at A 5050 is much more 

 pronounced than any of the others, but it is interesting to note that even 

 at the middle of this one there is considerable photographic blackening 

 produced by the 2-seconds exposure. The maxima and minima show 

 most distinctly in the two strips corresponding to 6-seconds and 8-seconds 

 exposure, respectively; which shows that at first the photographic action 

 in the maxima increases with time somewhat more rapidly than in the 

 minima. The A 5050 minimum is still visible in the 1-minute exposure, 

 while the others can be seen only with difficulty. In a full exposure (1 to 

 2 minutes) the photographic action in the red begins to shade off percep- 

 tibly at A 7250, but is still considerable as far as A 7500. B is a series of 

 exposures of the Seed L-Ortho-film; the successive times of exposure 

 being 2, 3, 4, 8, 15, and 30 seconds, and 1 minute, respectively. The cur- 

 rent in the filament, width of slit, and development were exactly the same 

 as used in making the negative for A. The strip corresponding to the 2- 

 seconds exposure shows maxima at A 4700 and A 5600, and a minimum at 

 A 5250, at the center of which the negative records no photographic action 

 whatever. The maximum at A 4700 in the 2-seconds exposure is slightly 

 stronger than the one at A 5600, while the reverse is the case in the strips 

 corresponding to exposure of 4 seconds or more. The action increases 



