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



portion of the opal ; but the general character of each individual stone is 

 well marked. 



No. 1 shows a single black band in the red. When properly in focus 

 this has a spiral structure. Examined with both eyes it appears in decided 

 relief, aud the arrangement of light and shade is such as to produce a 

 striking resemblance to a twisted column. 



No 2. gives an irregular line in the orange. Viewed binocularly, this 

 exhibits the spiral structure in a marked manner, the different depths and 

 distances standing well out ; upon turning the milled head 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 spec- 

 trum. It is not easy to retain the conviction that one is looking merely 

 at a band of deficient light in the spectrum, and not at a solid body, pos- 

 sessing dimensions and in actual motion. 



No. 3 has a line between the yellow and green, vanishing to a point at the 

 top, and near the bottom having a loop, in the centre of which the green 

 appears. Higher up, in the green, is a broad green band, indistinct on one 

 side and branching out in different parts. 



No. 4 has a broad, indistinct, and sloping band in the blue, and another, 

 still more indistinct, in the violet. 



No. 5 has a band in the yellow, not very sharp on one side, and some- 

 what sloping. Upon moving the opal sideways, it moves about from one 

 part of the yellow field to another. In one position it covers the line D, 

 and is opaque to the sodium-flame of a spirit-lamp. 



No. 6 shows a curiously shaped band in the red, very sharp and black, 

 and terminating in one part at the line D. In the yellow there is a black 

 dot. The spectrum of this opal showed by reflected light intensely bright 

 red bands, of the shape of the transmission bands. On examining this 

 opal with a power of 1 inch, in the ordinary manner, the portion giving 

 this spectrum appeared to glow with intense red light, and was bounded 

 with a tolerably definite outline. Without altering any other part of the 

 microscope, the prisms were then pushed in so as to look at the whole 

 surface of the opal through the prisms, but without the slit. The shape 

 and appearance of the red patch were almost unaltered ; and here and there 

 over other parts of the opal were seen little patches of homogeneous light, 

 which, not having been fanned out by the prisms, retained their original 

 shape and appearance. 



No. 7 shows a black patch in the red, only extending a little distance, 

 and a line in the yellow. On moving the opal the line in the red vanishes, 

 and the other line changes its position and form. 



No. 8 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 irregularly shaped and somewhat hazy 

 corkscrew, into the middle of the yellow. The drawing shows the appear- 

 ance of this band in two positions. 



