I20 C. >[. BROOMALL : 



were found to possess the shielding- action referred to in much 

 greater degree than others. With a stronger light than actu- 

 ally exists in the rainbow the shielding action would not be so 

 complete, as theoretically one thickness of mica could only 

 cause complete extinction for one particular color. 



On this occasion, although the primary bow was unusually 

 bright, the secondars^ rainbow was scarcely visible, so that it 

 was not possible to make use of the light from this source. 



As is well known, the primary bow is the result of two 

 refractions and one reflection in the rain-drops. The phenom- 

 enon in this case gives, therefore, an interesting example of 

 the production of a spectrum by refraction and the polariza- 

 tion of the light by reflection. 



In another experiment, a piece of Iceland spar, about an 

 inch and a half long and half an inch through, was the source 

 of both spectrum and polarization. This piece of calcite, 

 while quite clear, had become somewhat shattered by rough 

 usage and when looked at in certain directions exhibited 

 brightly colored, thin-film spectra. The planes producing 

 these colors were not parallel with the faces of the rhombohe- 

 dron, but were at all sorts of angles. The colors were visible 

 upon viewing the cr3'Stal from several different directions and 

 in every case were found to exhibit polarization. A Nichol 

 prism held in the hand in one position would allow the colors 

 to be seen in all their brightness, while when the prism was 

 rotated ninety degrees all light disappeared. An analysis of 

 the paths of the light rays in this experiment would be dif- 

 ficult and would no doubt show the action to be different for 

 different positions of the crystal. When the light first enters 

 the calcite it probably suffers the usual polarization and is 

 divided into the ordinary and extraordinary rays vibrating at 

 right angles to each other. What would happen to these rays 

 at the surfaces of the minute cracks is problematical. Among 

 other actions there might be unequal reflection of the two 

 rays, or there might be recombination and color-effects by 

 extinction : again, there might be the independent production 



