1022 Prof. Wood and M. L. Dunoyer on Separate Excitation 



of light and the consequent slight obliquity o£ some of the 

 pencils, the polarizing separator distributes the monochro- 

 matic light in bands, resembling interference bands. 



If the quartz plate is turned slowly the bands file across 

 the strip of magnesia. The brighter bands represent D 2 , and 

 the feebler D ± . If the two sodium lines were or equal 

 intensity the bands would of course be invisible. 



If, now, these bands are not parallel to the strip, but cut 

 across it obliquely, it is clear that the illumination may be 

 by D 2 a t the top, by D 1 and D 2 at the middle, and by JDi at 

 the bottom. In this way it would be possible to obtain, 

 with one exposure, three types of excitation. 



This method was not used, however, and the quartz block 

 was adjusted by tipping it slightly in the direction of the 

 source or image until the bands were vertical and the illu- 

 mination constant and of the same type (D 2 ) all along the 

 slit of the spectroscope. The bulb was now rotated until the 

 strip of magnesia fell into coincidence with one of the lateral 

 images (D : ), the narrow horizontal strip of magnesia cutting 

 across the D 2 image. The burner which heated the bulb 

 was now lighted, and as soon as the superficial resonance 

 reached its full intensity the exposure was commenced. 

 Panchromatic plates (W ratten & Wainwright) were cut 

 into small squares large enough to cover the eyepiece 

 tube of the spectroscope, and were held in place against 

 the latter by two thicknesses of black cloth and a rubber 

 band. This method permits of very accurate focussing 

 and is extremely convenient if the spectroscope has no 

 plate-holder. 



TVe first used a spectroscope furnished with cinematograph 

 lenses of large aperture (F4), which was loaned to us 

 by M. Debierne. With a Rutherford compound prism this 

 instrument resolved the D lines on the photographic plate if 

 the slit was made exceedingly fine. The first photograph, 

 which was made of the resonance excited by the light of the 

 D 2 line, showed no trace of D^ The exposure was of three 

 hours' duration. A second attempt with a five-hour exposure 

 showed a trace of D 1? but examination of the exciting light 

 showed that D x was present. This was found to be due to 

 the rise of temperature (5°) of the room during the exposure, 

 the polarizing separator being fairly sensitive to temperature 

 changes. The first pictures were made before the expedient 

 of the narrow horizontal strip of magnesia had been adopted. 

 This proved to be a great convenience, for a record was left 

 of the integrated condition of the exciting line for the whole 

 exposure. 



