Mr. W. Crookes on some Optical Phenomena of Opals. 389 



but the flashes of coloured light are better seen and examined when 

 the top and bottom of the gems are ground and polished flat and 

 parallel. 



A good opal is not injured by moderate heating in water, soaking 

 in turpentine, or heating strongly in Canada balsam and mounting 

 as a microscopic slide. 



By the kindness of Mr. W. Chapman, of Frith Street, Soho, and 

 other friends, I have been enabled to submit some thousands of opals 

 to optical examination ; and from these I have selected about a dozen 

 which appeared worthy of further study. 



If an opal which emits a fine broad crimson light is held in front 

 of the slit of a spectroscope or spectrum-microscope, at the proper 

 angle, the light is generally seen to be purely homogeneous, and all 

 the spectrum that is visible is a brilliant luminous line or band, 

 varying somewhat in width and more or less irregular in outline, but 

 very sharp, and shining brightly on a perfectly black ground. If, 

 now, the source of light is moved, so as to shine into the spectrum- 

 apparatus through the opal, the above appearance is reversed, and 

 we have a luminous spectrum with a jet-black band in the red, iden- 

 tical in position, form of outline, and sharpness with the luminous 

 band previously observed. If instead of moving the first source of 

 light (the one which gave the reflected luminous line in the red) an- 

 other source of light be used for obtaining the spectrum, the two ap- 

 pearances, of a coloured line on a black ground, and a black line on 

 a coloured ground, may be obtained simultaneously, and they will be 

 seen to fit accurately. 



Those parts of the opal which emit red light are therefore seen to be 

 opaque to light of the same refrangibility as that which they emit ; 

 and upon examining in the same manner other opals which shine 

 with green, yellow, or blue light, the same appearances are observed, 

 showing that this rule holds good in these cases also. It is doubtless 

 a general law, following of necessity the mode of production of the 

 flashes of colour. 



Having once satisfied myself that the above law held good in all 

 the instances which came under my notice, I confined myself chiefly 

 to the examination of the transmitted spectra, although the following 

 descriptions will apply equally well, mutatis mutandis, to the re- 

 flected spectra. The examinations were made by means of the spec- 

 trum-microscope, which instrument is peculiarly adapted to exami- 

 nations of this sort, both on account of the small size of the object 

 which can be examined in it, and also as it permits the use of both 

 eyes in viewing the spectrum. 



The following is a brief description of some of the most curious 

 transmission spectra shown by these opals. The accompanying 

 figures, drawn with the camera lucida, convey as good an idea as pos- 

 sible of the different appearances. The exact description will of 

 course only hold good for one 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 



