390 Royal Society : — 



in decided relief, and 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 spectrum. 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, possessing 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 

 somewhat 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 or- 

 dinary 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 homo- 

 geneous 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 dis- 

 tance, 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 appearance of this band in two positions. 



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 azi- 

 muth, whilst in the field of the instrument, the lines can be made to 



