180 FRANCIS DEAS ON SPECTRA FORMED BY THE PASSAGE OF 



duce in the spectrum, only blacker and better denned than these are ever 

 seen. 



The following is the mode in which the band makes its appearance. As the 

 zero point is passed, the light first makes its appearance in the green of the 

 spectrum, from which point, as the selenite is rotated, the light opens out in 

 both directions. When the light reaches the red, the black band makes its 

 appearance, and attains a maximum blackness when the selenite is at 45°, viz., 

 when, without the use of the dispersion prisms, the field would contain green 

 light of maximum brightness. When this point of revolution is passed, the 

 band again fades, the spectrum becomes obscured at each end, the darkness 

 creeping in towards the green, till at 90° the spectrum has again vanished. The 

 same phenomena recur at each quarter of a revolution. 



Let the Nicol's prisms now be set with their axes parallel, and the same 

 selenite rotated on the stage as before. The result is what we should be led to 

 expect from the last experiment. At the zero points the selenite exercises no 

 influence, and we have a continuous ordinary spectrum. As a zero point is 

 passed, however, a dark band makes its appearance, but this time in the green 

 rays. The band is at first faint and nebulous, but becomes blacker and sharper 

 as the stage is rotated, till at 45° it attains its maximum. The spectrum in this 

 experiment never vanishes, but is apparently quite continuous throughout, save 

 for the appearance of the black band. 



Lastly, let the selenite be fixed at 45° from the zero point, and the analyser 

 rotated. We have now a combination of the two previous experiments. The 

 band in the red appears alternately with the band in the green at each quarter 

 revolution, the former being at its maximum when the axes of the prisms are 

 perpendicular, the letter when these axes are parallel.* 



The above are the appearances which present themselves in the case of most 

 films of selenite of a medium thickness. In some cases, however, two, or even 

 three, black bands occur simultaneously, these being always followed by as 

 many complementary bands, when the analyser is turned through 90°. The 

 number of bands can generally be multiplied by using two or more films in 

 combination, and the appearances can be still further varied by changing the 

 degree of inclination of the axes of the two films to one another. If the two 

 films are placed with their similar axes coincident, we obtain, of course, the 

 spectrum appropriate to a film equal to the sum of the thicknesses of the two 

 films, while, if dissimilar axes are superposed, the spectrum is that due to the 

 difference of the same. I have two films which, when properly combined, give 

 no less than six well-marked bands simultaneously. 



* If the axis of the selenite makes a greater or less angle than 45° with the plane of polarisation, 

 the result is that while the same band still recurs after 180° of a revolution of the analyser, the com- 

 plementary band is no longer separated from it by 90°, but by a greater or less angle. 



