1869.] On the Spectral Phenomena of Opals. 485 



turning the opal round and observing it from difierent directions, it 

 is generally possible to get a position in which it shows no colour 

 whatever. Viewed by transmitted hght, opals appear more or less 

 deficient in transparency, and have a slight greenish, yellow, or 

 reddish tinge. 



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 hght are better seen and examuied 

 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 microscoj)ic slide. 



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 briUiant luminous hne or 

 band, varying somewhat in width and more or less irregular in 

 outline, but very sliarj), and shining brightly on a perfectly black 

 ground. If, now, the source of hght is moved so as to shine into 

 the spectrum apparatus tlirougli the opal, the above appearance is 

 reversed, and we have a luminous sj)ectrum 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 hght be used for obtaining the spectrum, 

 the two ajjpearances of a coloured hne on a black ground and black 

 line on 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 refrangibihty which they emit ; 

 and upon examining in the same manner other opals which shine 

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

 shoAving that this rule holds good in these cases also. It is doubt- 

 less 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 reflected spectra. 



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

 transmission spectra shown by these opals. The chromo-litho- 

 graphs forming the frontispiece, taken from drawings with the 

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

 appearances. The exact description will, of course, only hold good 



