388 Royal Society: — 



O'Ol to O'OOl inch, according to the actual width of the slit), it is 

 evident that the spectrum of the smallest objects can be examined. 

 If some blood is in the field, it is easy to reduce the size of the image 

 of the slit to dimensions covered by one blood-disk, and then, by 

 pushing in the prisms, to obtain its spectrum. 



Tf the object under examination will not transmit a fair image 

 of the slit (if it be a rough crystal of jargoon for instance), it must 

 be fixed in the universal holder beneath the slit and the light con- 

 centrated on it before it reaches the slit. If the spectra of opaque 

 objects are required, they can also be obtained in the same way, the 

 light being concentrated on them either by a parabolic reflector or by 

 other appropriate means. 



By replacing the illuminating lamp by a spirit-lamp burning with a 

 soda-flame, and pushing in the spectrum-apparatus, the yellow sodium- 

 line is seen beautifully sharp ; and by narrowing the slit sufficiently 

 it may even be doubled. Upon introducing lithium- or thallium-com- 

 pounds into the flame, the characteristic crimson or green line is 

 obtained ; in fact so readily does this form of instrument adapt 

 itself to the examination of flame-spectra, that for general work I 

 have almost ceased to use a spectroscope of the ordinary form. The 

 only disadvantage I find is an occasional deficiency of light ; but by 

 an improved arrangement of condensers I hope soon to overcome this 

 difficulty. 



" On some Optical Phenomena of Opals." By William Crookes, 

 F.R.S. &c. 



When a good fiery opal is examined in day-, sun-, or artificial 

 light, it appears to emit vivid flashes of crimson, green, or blue light, 

 according to the angle at which the incident light falls, and the rela- 

 tive position of the opal and the observer ; for the direction of the 

 path of the emitted beam bears no uniform proportion to the angle 

 of the incident light. Examined more closely, the flashes of light 

 are seen to proceed from planes or surfaces of irregular dimensions 

 inside the stone, at different depths from the surface and at all angles 

 to each other. Occasionally a plane emitting light of one colour 

 overlaps a plane emitting light of another colour, the two colours 

 becoming alternately visible upon slight variations of the angle of the 

 stone ; and sometimes a plane will be observed which emits crimson 

 light at one end, changing to orange, yellow, green, &c, until the 

 other end of the plane shines with a blue light, the whole forming 

 a wonderfully beautiful solar spectrum in miniature. I need scarcely 

 say that the colours are not due to the presence of any pigment, 

 but are interference colours caused by minute striae or fissures lying 

 in different planes. By turning the opal round and observing it 

 from different directions, it is generally possible to get a position in 

 which it shows no colour whatever. Viewed by transmitted light, 

 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 ; 



