8 The Ethereal Hypothesis of Light. [Jan., 



surface, which is then opahne (Fig. 6a), and the imbedded metalhc 

 crystals are imisible. As to the dimensions of my httle preparation, 

 it is difficult to form a correct idea of them. In the Plate (Fig. 6 

 and 6a) the whole object is magnified about 5 diameters, or about 

 25 times in superficial area : but from the mode in which it is 

 mounted between sheets of glass above and below (the former being 

 a combination of two shps, flint and crown, I beheve), it is impos- 

 sible to form an accurate estimate of its thickness. In conjunction 

 with a friend, an experienced microscopical observer, I have, how- 

 ever, tried to form an approximate idea, and with a magnifying 

 power of 56 diameters, it aj)pears about -.V of an inch thick, there- 

 fore in reahty it may be from T-frrnr to -nrinr of an inch. This is a 

 verv rude mode of arriving at its thickness, but it would be quite 

 useless even to guess at the degree of tenuity of the contained 

 crystals or plates. They arc embedded at various depths in this 

 thin shaving of mineral, showing no indications of an edge, and are 

 sometimes at such relative distances below each other, that the focus 

 requires to be considerably changed to bring one after the other 

 into full view. Occasionally they are superposed one above 

 another with a space intervening. I have examined some of the 

 individual crystals with powers of 200, 270, 540, 900, and about 

 1300 diameters, and notwithstanding their extreme tenuity, I have 

 not been able to detect the least apj^earance of structure, or breach 

 of continuity in the uniformly flat orange-yellow surface which they 

 present to the eye by transmitted light. Where a crystal happens 

 to be imperfect, its broken edge examined with a high power, re- 

 sembles torn paper, but has no indication of geometrical symmetry. 



As already stated, these crystals, which by transmitted light are 

 a uniform orange, by incident light rejled aU the colours of the 

 spectrum, each crystal reflecting usually one colour, but often the 

 same is variously tinted. In Fig. 7 I have attempted to represent a 

 few of them, but no idea can be formed of then' brilliancy unless ihoj 

 are seen under a good bright light in nature. Sometimes (as in the 

 three upper crystals in the figure) the same crystal reflects a metallic 

 lustre in one position, and exhibits the orange transparency in 

 another. Sometimes again, a sharp line divides a crystal in two, and 

 if it be moved round, each division will, by turns, present the trans- 

 parent orange and the reflected hght. This is doubtless owing to the 

 light being reflected from some neighbouring crystal or from one of 

 the fissures of the felspar, and passing upwards through the whole 

 or part of the crystal, as in the case where transmitted hght is used. 



The matrix of felspar partially depolarizes polarized light, but 

 the latter has not the least effect upon tlie imlx'ddc^l crystals. 



Before considering the bearing of these phenomena upon our 

 inquiry, let us briefly refer to another, in which the experimenter 

 calls into action the force of which he at the same time observes the 



