486 On the Sped ml Phenomena of Opals. [Oct., 



for one portion of the opal, but the general character of each 

 individual stone is well marked. 



The simplest form of band which is met with is shown in 

 Spectrum 1, which represents the spectrum after passing through 

 one of my experimental opals. It 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. 



Spectrum 2 shows a band of a very remarkable character ; it is 

 broad and black, and cuts diagonally across the green, touching the 

 blue at the top and the yellow at the bottom. 



Spectrum 3 is somewhat similar ; it shows a broad indistinct 

 diagonal band in the blue, and another still more indistinct in the 

 violet. 



So far the bands have been simple lines, and the only pecuharity 

 has been the diagonal character of some of them. I now wish to 

 draw attention to a most remarkable phenomenon presented by 

 some opals, and that is the production in their transmission spectra 

 of irregularly shaded lines, which when examined in the binocular 

 spectrimi instrument appear distinctly spiral, and, on moving the 

 opal, roll over on their axis from one part of the spectrum to 

 another. Spectrum 4 is an example of this kind; it shows an 

 irregular line in the orange. Viewed binocularly this exhibits a 

 spiral structure in a marked manner, the diflerent depths and 

 distances standing well out : upon turning the milled edge of the 

 stage adjustment, so as to carry the opal slowly from left to right, 

 the spiral line is seen to revolve and roll over, altering its shape 

 and position in the spectrum. It is not easy to retain the conviction 

 that one is looking merely at a band of deficient light in the spec- 

 trum, and not at a solid body possessing dimensions and in actual 

 motion. 



Spectrum 5 shows the most striking example of a spiral rotating 

 line which I have yet met with. On moving the opal sideways the 

 line is seen to start from the red and roll over, like an ii-regulai-ly 

 shaped and somewhal hazy corkscrew, uito the middle of the yellow. 

 The drawing shows the appearance of this band in two positions. 



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 aj)pearance of a luminous line, slowly moving across 

 I he 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 lumi- 

 nous bands, by illuminating the opals with rcflcctod light. They are, 

 however, more difficult to see, for the coloured light is only emitted 

 at a particular angle, whilst the special oi>acity to the ray of the same 

 refrangibility as the emitted ray holds good f()r diflerent angles. 



