1869.] Mr. W. Crookes on some Optical Phenomena of Opals. 449 



In order to better adapt them to the purposes of the jeweller, opals are 

 almost always polished with rounded surfaces, back and front ; 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, identical 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) 

 another source of light be used for obtaining the spectrum, the two appear- 

 ances, 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 accu- 

 rately. 



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 ex- 

 amining 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 in- 

 stances which came under my notice, I confined myself chiefly to the 

 examination of the transmitted spectra, although the following descrip- 

 tions will apply equally well, mutatis mutandis, to the reflected spectra. 

 The examinations were made by means of the spectrum-microscope a de- 

 scription of which I have just had the honour of sending to the Society. 

 This instrument is peculiarly adapted to examinations 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 transmis- 

 sion spectra shown by these opals. The accompanying figures, drawn with 

 the camera lucida, convey as good an idea as possible of the different ap- 

 pearances. The exact description will of course only hold good for one 



