232 REPORTS ON INVESTIGATIONS AND PROJECTS. 



faint, which shows that the power of rotating the plane of polarization of 

 the transmitted light is only possessed by certain electrons of each group. 

 The results obtained from these experiments are being compared with the 

 observations made of the fluorescence produced by monochromatic stimula- 

 tion. It has been found, in addition, that not only iodine, but bromine, 

 potassium, and the colored oxides of nitrogen give magnetic rotation spec- 

 tra. The bright-line rotation spectrum of iodine has been photographed 

 with the 15-foot grating, in coincidence with the absorption spectrum, and 

 a study is in progress of the relations between the two. The color of the 

 transmitted light is with iodine bright green, with potassium yellow, and 

 with the nitrogen oxides blue-green. The yellow seen with potassium is 

 due to traces of sodium, which give powerful rotation at the D lines. The 

 spectra are all discontinuous, and their study is in progress. The fluo- 

 rescence spectrum and the temperature emission spectrum of iodine have 

 both been photographed and shown to be discontinuous. 



LAMBERT'S LAW AND FLUORESCENCE. 



Lambert's law states that the intensity of the radiation emitted by self- 

 luminous surfaces of solids and liquids varies in intensity with the cosine 

 of the angle of emission. This is an empirical law based upon the observa- 

 tion that the sun or a red-hot iron ball appears as a uniformly illuminated 

 disk. It is not obeyed by non-absorbing gases, as is shown by the fact that 

 a gas flame has a greater intrinsic intensity when viewed edgewise than 

 when observed from the side. An accidental observation made last year 

 that the fluorescent surface of a prism of crown glass illuminated with 

 ultra-violet light increased enormously in intensity as it was foreshortened 

 by turning the prism made it seem worth while to ascertain whether the 

 radiation in this case obeyed the same law as that of the gas flame. A pho- 

 tometric study of the luminosity of the surface at various angles of emission 

 has been made, with the result that it has been shown that the intrinsic 

 intensity of a fluorescent surface viewed, so to speak, within the medium 

 increases in proportion as the apparent width of the surface is diminished 

 by foreshortening, or, in other words, the total amount of light given off is 

 independent of the direction, as is the case with a surface emitting X-rays. 



INTERFERENCE COLORS OF CRYSTALS OF CHLORATE OF POTASH. 



Thin flakes of chlorate of potash, obtained by cooling a hot saturated solu- 

 tion, often show interference colors of extraordinary brilliancy and purity. 

 These colors were studied by Stokes and Lord Rayleigh, and shown to be 

 often nearly monochromatic — i. e., the reflected light, when examined with 

 a spectroscope, was found to consist of one or more extremely narrow bands 

 in contrast to the broad and ill-defined bands shown in the colors of thin 



