1869.] On the Spectral Phenomena of Opals. 483 



the body of the microscope does not interfere with the working of 

 the instrument in the ordinary manner. The length of the tube is 

 increased 1 or 2 inches, and a httle additional rackwork may in 

 some instruments be necessary when using object-glasses of low 

 power. The stereoscopic effect, when the Wenham prism is put 

 into action, does not appear to be interfered with. 



For ordinary work both these additions may be kept attached 

 to the microscope, the prisms being pushed to the side of the prism- 

 box, and the large aperture d being brought into the centre of the 

 substage. When it is desired to examine the spectrum of any 

 portion of an object in the field of view, all that is necessary is to 

 push the slit into adjustment with one hand, and the prisms with 

 the other. The spectrum of any object which is superposed on the 

 image of the slit is then seen. 



When the spectrum of any substance is in the field and a 

 double-image prism is introduced, two spectra are seen, one above the 

 other, oppositely polarized, the same lines being continued through 

 the two; and the variations in the absorption-lines such as are 

 shown by didymium, jargonium, &c., are at once seen. 



If the substance under examination is dark coloured, or the 

 illummation is not brilliant, it is best not to divide the light by 

 means of the Wenham prism at n, but to let the whole of it pass 

 up the tube to one eye. If, however, the light is good, a very great 

 advantage is gained by throwing the Wennam prism into adjustment, 

 and using both eyes. The aj^pearance of the spectrum, and the 

 power of grasping faint hues, are incomparably superior when both 

 eyes are used ; whilst the stereoscopic efi'ect it confers on some 

 absorption and interference spectra (especially those of opals) seems 

 to throw entirely new light on the phenomena. No one who has 

 worked with a stereoscopic spectrum apparatus would willingly 

 return to the old monocular spectroscope. 



If the illumination in this instrument is taken from a white 

 cloud or the sky, Fraunhofer's lines are beautifully visible, and 

 when using direct sunlight they are seen with a perfection which 

 leaves httle to be desired. The dispersion, being four or five times 

 greater, is sufiicient to cause the spectrum to fill the whole field of 

 the microscope, instead of, as in the ordinary instrument, forming a 

 small portion of it ; whilst owing to the very perfect achromatism 

 of the optical part of the microscope aU the Imes from b to g are 

 practically in the same focus. 



As the only portion of the object examined is that part on 

 which the image of the slit falls, and as this is very minute (varying 

 from 0-01 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 



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