1869.] The EtJiereal Hypothe&is of Light. 7 



After reading the account of Faraday's beautiful and exhaustive 

 experiments,* of which I have only referred to one series, I should 

 have despaired of being able to add any information that would 

 elucidate our inquhy had not nature herself prepared a beautiful 

 microscopical object, which, in common with many friends, I have 

 examined with undiminished pleasure and admiration for nearly 

 twelve years. 



It is called the " Sonnenstein," or Sun Stone, from its peculiar 

 brilliancy, and is found in Arendal in Norway. Its brightness is 

 due to innumerable minute metaihc crystals imbedded in a matrix 

 of a translucent substance of a spar-like nature. Examined with a 

 low power by transmitted light, it resembles a colourless transparent 

 fragment of glass or spar, containing irregularly-shaped pale, 

 orange, and red translucent crystals (Fig. 6) ; where these are 

 superposed one above another, they assume a brighter hue; but 

 when viewed by incident light an almost miraculous transformation 

 takes place, some of the crystals appearing bright blue, others pre- 

 senting every colour of the spectrum, and if the object be turned 

 slowly round the same crystals reflect diiferent hues as they revolve.f 

 The transparency of the matrix is due to its extreme tenuity (the 

 object having been cut with the aid of a mechanical contrivance, by 

 the late Dr. Oschatz of Berlin), and viewed with a higher power 

 under incident hght in one position, the hght is reflected from its 



* I have also considered those of Professor Tyndall, just published in the 

 ' Proceedings of the Eoyal Society,' and fully reported in our Chronicle of Physics ; 

 but although they are very interesting, I do not at present see anything in them to 

 throw fresh light on our inquiry. 



t My correspondent, Blr. T. Rudler, of the Museum of Practical Geology, gives 

 me the following account of the " Sonnenstein." 



"The mineral called by the Germans ' Sonnenstein,' by the French ' Pierre de 

 Soleil,' and by the English ' Sun-stone,' is a variety of Oligoclase-felspar, originally 

 discovered at Archangel, but now found chiefly in Norway. It exhibits a beautiful 

 spangled appearance, somewhat resemblmg that of Aventurine, and hence it has 

 been called Aveuturine-felspar. This appearance is apparently due to the reflection 

 ef light from the walls of minute fissures traversing the stone, and also to the 

 presence of small six-sided plates wliich are usually disseminated through the 

 mineral. What tliey really are, is ditTicult to say. Scheerer regarded them as 

 crystals of specular Iron Ore (anhydrous Peroxide of Iron), and Oschatz confirmed 

 this pbservation, but Kenngott will have it that they are magnetic pyrites ( Pyr- 

 rhotine). Some, I believe, regard them as titaniferous iron ore, whilst others refer 

 them to the species Gothite. Formerly they were thought to be little scales of Mica. 



" I ought, perhaps, to say there is some little confusion in the use of the word 

 ' Sun-stone,' as a few writers have applied it to an opalescent potash-felspar, or 

 Adularia. The beautiful colours exhibited under the microscope by the embedded 

 crystals are due, I should think, rather to their extreme thinness than to any 

 coloui' inherent in the crystals." 



To this account of Mr. Eudler, I would add that from the analogy between them 

 and the iridescent butterfly scales (Figs. 2 and 2a in my Plate) which are red by trans- 

 mitted, and blue by incident light, 1 have no doubt that their V)rilliant colours are due 

 to refltxion from their surface. In my specimen there are scales which are opaque 

 by transmitted light and which nioie nearly resemble a crumpled fragment of 

 metallic tinsel than a flat geometrical crystal ; indeed the oystals are very irregu- 

 lar and not often six-sided. 



