448 Mr. W. Crookes on some Optical Phenomena of Opals. [May 27, 



0*001 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. 



If 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 concentrated on it be- 

 fore 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 compounds 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 spectro- 

 scope 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. 



V. " On some Optical Phenomena of Opals." 

 By William Crookes, F.R.S. &c. Received April 23, 1869. 



"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 relative 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 sur- 

 faces 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 wonder- 

 fully 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 trans- 

 parency and have a slight greenish yellow or reddish tinge. 



