392 Royal Society .— 



alter in inclination until they are seen to slope in the opposite di- 

 rection. 



No. 1 1 gives another illustration of a diagonal line, across the yel- 

 low 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 sideways. 



It is scarcely necessary to say that the colour of the moving lumi- 

 nous line varies with the part of the spectrum to which it belongs. 

 The appearance 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, how- 

 ever, 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 some- 

 what 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 ; this 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 spectrum-microscope, the slit 

 will be successively illuminated with light of gradually increasing 

 refrangibility, and the appearance of a moving luminous line will be 

 produced ; and if transmitted light is used for illumination, the re- 

 versal 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 describe it in words. 



It is probable that similar phenomena may be seen in many, if not 

 all, bodies which reflect coloured light after the manner of opals. A 

 magnificent specimen of Lumacelli, or Fiery Limestone, from Italy, 

 kindly presented to me by my friend David Forbes, shows two sharp 

 narrow and parallel bands in the red. I have also observed similar 

 appearances in mother-of-pearl. The effects can be imitated to a 

 certain extent by examining " Newton's rings," formed between two 

 plates of glass, in the spectrum-instrument. 



