452 Mr. W. Crookes on some Optical Phenomena of Opals. [May 27, 



No. 9 is one of the most curious. A broad black and sharp band 

 stretches diagonally across the green, touching the blue at the top and the 

 yellow at the bottom. 



No. 10 gives a diagonal band, wide, but straight, and tolerably sharp 

 across the green. By rotating these opals, 9 and 10, in azimuth, whilst 

 in the field of the instrument, the lines can be made to alter in inclination 

 until they are seen to slope in the opposite direction. 



No. 1 1 gives another illustration of a diagonal line, across the yellow and 

 green, not extending quite to the top. 



No. 12 is one of the best examples I have met with of a narrow, straight, 

 and sharply cut line. It is in the green, and might easily be mistaken for 

 an absorption-band caused by an unknown chemical element. 



Other opals are exhibited which show a dark band travelling along the 

 spectrum, almost from one end to the other, as the opal is moved side- 

 ways. 



It is scarcely necessary to say that the colour of the moving luminous 

 line varies with the part of the spectrum to which it belongs. The ap- 

 pearance of a luminous line, slowly moving across the black field of the 

 instrument, and assuming in turn all the colours of the spectrum, is very 

 beautiful. 



All these black bands can be reversed, and changed into luminous bands, 

 by illuminating the opal with reflected light. They are, however, more 

 difficult to see ; for the coloured light is only emitted at a particular angle, 

 whilst the special opacity to the ray of the same refrangibility as the 

 emitted ray holds good for all angles. 



The explanation of the phenomena is probably as follows : — In the case 

 of the moving line, the light-emitting plane in the opal is somewhat broad, 

 and has the property of giving out at one end, along its whole height and 

 for a width equal to the breadth of the band, say, red light ; tbis merges 

 gradually into a space emitting orange, and so on throughout the entire 

 length of the spectrum, or through that portion of it which is traversed 

 by the moving line in the instrument, the successive pencils (or rather 

 ribbons) of emitted light passing through all degrees of refrangibility. 

 It is evident that if this opal is slowly passed across the slit of the spec- 

 trum-microscope, the slit will be successively illuminated with light of 

 gradually increasing refrangibility, and the appearance of a moving lumi- 

 nous line will be produced ; and if transmitted light is used for illumination, 

 the reversal of the phenomena will cause the production of a black line 

 moving along a coloured field. A diagonal line will be produced if an opal 

 of this character is examined in a sloping position. 



The phenomenon of a spiral line in relief, rolling along as the opal is 

 moved, is doubtless caused by modifying planes at different depths and 

 connected by cross planes ; I can form a mental picture of a structure 

 which would produce this effect, but not clear enough to enable me to de- 

 scribe it in words. 



