484 On the Spedrcd Fhenmnena of Opah. [Oct., 



blood-disc, and then, by pushing in the prisms, to obtain its 

 spectrum. 



If the object under examination will not transmit a fair image 

 of the sUt (if it be a rough crystal of jargon, for instance), it must 

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

 centrated on to it before it reaches the slit. If the reflected 

 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-lamj) 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 mto 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 additional facilities afforded by the use of this instrument 

 have led the writer to the discovery of one or two facts con- 

 nected with the action of different bodies on the rays of the 

 spectrum; and it has been considered that these may be of suf- 

 ficient interest to the readers of the ' Quarterly Journal of Science * 

 to be described somewhat ia detail and illustrated by a few chromo- 

 lithographs. 



The mineral known as Oj^a? has long been prized as a gem, on 

 account of the extraordinary beauty of its colours, but the phe- 

 nomena underlying these flashes of colour have not been submitted 

 to strict scientific examination. 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 tlae opal and the 

 observer ; for the direction of the jjath 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 fonuing 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 



