Royal Microscojncal Society. 115 



one of the tubes. This is because of the way in which the double- 

 image prisms are ground ; that is, equal to one Nicol's prism turned 

 round 90° ; hence you cannot get the same colour in each field 

 unless, as I say, a Nicol's prism is placed in one of the tubes or 

 on the top of the eye-piece. 



I think that this binocular eye-piece might be used with ad- 

 vantage in the telescope, for by its means we could tell whether the 

 object examined shone by its own Hght or by reflected light. 



V. — On Crystalline Forms modified hy Colloid Silica. 

 By Heney James Slack, F.G.S., Sec. E.M.S. 



{Tahen as read before the Koyal Microscopical Society, Feb. 8.) 

 Plates LXXVII. and LXXVm. 

 The remarks in this paper must be regarded as in continuation of 

 those previously communicated to the Society. The nature of the 

 action of colloid sihca in modifying the patterns obtained in preparing 

 various crystals for observation with the microscope and polarizing 

 apparatus, has already been explained, and we may at once proceed 

 to the consideration of the figures in the accompanying Plates. 



Fig. 1 represents a portion of a pattern obtained by heating 

 and quickly drying a drop of a strong solution of cadmic chloride 

 prepared with distilled water containing colloid silica. The whole 

 pattern is approximately circular — what is shown is included 

 between two radii and a curved line near the outer circumference, 

 about 1 — 10". In this slide crystallization has suffered extreme 

 interruption, and the result is a deposition of the salt in a form 

 rudely, but strikingly resembling the patterns of sections of certain 

 echinus spines. In the centre are some clear cellular-looking spaces, 

 and on all sides proceed radial lines, with concentric lines at inter- 

 vals. Fig. 10 shows the same salt in a crystalline flower pattern. 



Fig. 2. Hippuric acid prepared with the silica. Here we see 

 the action of radial and tangential forces. The spirals, in bands from 

 1 — 170" to 1 — 200", appear to be thickened and shghtly-raised 

 portions of the general structure, which is a modification of the 

 well-known grouping of radiating needles into circular j)atterns. 



The strong lines show silica cracks. 



Fig. 3, hippuric acid and silica, exhibits graceful floral curves, 

 in which the angular character of simple and pure crystaUization 

 is modified into organic outlines like those of exquisitely-shaped 

 flowers and foliage. High-power views (|th and A or B eye-piece) 

 exhibit multitudes of minute markings, cracks, and modifications of 

 level. Size from about 1 — 30" to 1 — 100". 



Fig. 4 is taken from a shde of hippuric acid and silica. This 

 slide contains circular groups of various sizes with curiously-frilled 



