389 



certain Crystals/' by Henry Fox Talbot, Esq., F.R.S., was resumed 

 and concluded. 



In this memoir the author gives an account of the optical proper- 

 ties of certain minute crystals, obtained by the evaporation of a solu- 

 tion of borax in phosphoric acid, exhibited when they are examined 

 by means of the polarizing microscope. The field of view is then 

 seen covered with minute circular spots, each composed of a close 

 assemblage of delicate acicular crystals, radiating from the centre ; 

 together with other circular bodies, in which this disposition is not 

 observable, on account of the close union of the component crystals, 

 which, producing optical contact, gives perfect transparency to the 

 whole mass. When the field of view is rendered dark by the rectan- 

 gular crossing of the polarizing laminae, each of these little circles 

 becomes luminous, and exhibits a well-defined dark cross, dividing 

 its area into four equal sectors. These crosses have a similar position 

 in all the circles; and their direction remains unaltered when the 

 crystals are turned round in their own plane, by causing the plate of 

 glass, on which they are placed, to revolve. 



On examining the larger circles with a high magnifying power, and 

 under favourable circumstances of illumination, the author observed 

 upon each a series of coloured concentric rings : but the number as 

 well as the colour of these rings varies in different crystals. The in- 

 nermost ring is deeply coloured, or black; and incloses a central 

 space of white light, which is traversed by the arms of the cross, in- 

 tersecting in the centre. This part of the cross, which stands within 

 the innermost ring, is beautifully defined, and perfectly black. The 

 whole system of phenomena are exactly analogous to that exhibited 

 by uniaxal crystals ; and corresponds still more closely with those dis- 

 covered by Sir David Brewster in spheres of glass, the density of 

 which had been rendered variable from the centre to the surface by 

 immersion in heated oil; excepting that the miscroscopic crystals 

 here described are possessed of a far more intense polarizing energy. 

 The author thinks it probable that the phenomena are in both cases 

 produced by similar conditions of density j which, in a circular mass 

 formed by the aggregation of needle-shaped crystals radiating from 

 a common centre, it is natural to suppose would rapidly increase 

 from the circumference to the centre. By watching the progress 

 of crystallization he ascertained that this was, in fact, the mode in 

 which the crystals are constructed : for they frequently appeared, at 

 first, in the form of lengthened prisms, which subdivided themselves 

 at both ends into an immense multitude of divergent fibres, like those 

 of a brush ; apparently repelling each other as they extended in length, 

 and occupying spaces corresponding to two opposite sectors of a cir- 

 cle ; until, by spreading still farther in breadth, their edges united, 

 and filled the whole of a circular area. In all the stages of this pro- 

 cess, the formation of the black cross may be seen to keep pace with 

 the developement of the crystal, until perfectly displayed on the com- 

 pletion of the crystalline structure. The author notices the analogy 

 which this structure presents with that of the crystalline lens of the 

 cod fish, as has been lately described by Sir David Brewster 5 and 



