20 Mr. Talbot on Analytic Crystals. 



Dissolve horacic acid in boiling water, and put a drop of 

 the solution between two plates of glass. It will immediately 

 crystallize in irregular forms (Plate I. upper figure.). 



Place it on the stage of the polarizing microscope using the 

 liolari^^er only, and no analyser. 



It will then be seen that these crystals (which are floating 

 in water) present themselves under two aspects, according to 

 the direction in v-^hich they lie. Those which lie one way ap- 

 pear with all their outlines strongly defined and extremely 

 dark. Those which lie the other way (or vertically to the 

 first) appear on the contrary as faint as possible. In our 

 plate it was necessary to represent them as tolerably distinct, 

 but in fact it often happens that their faintness is so extreme 

 that they cease to be visible. This circumstance affords an 

 example of one of the most curious things in optical science: 

 for if one of the crystals has been carefully placed in this 

 position, and a person be then desired to look into the micro- 

 scope, he will confidently assert that there is no object in the 

 field of view. If then the polarization of the light be reversed 

 by turning the polarizer round 90°, the crystal will appear to 

 start into existence, and become not only as dark as those 

 represented in the figure, but even darker, or perhaps en- 

 tirely opake. 



Without entering into the theory of the matter here, it 

 appears to me to result from this, that, of the two refractive 

 indices of boracic acid this j^'epared^ one is the same with the 

 refractive index of water. 



If now, the appearance of things being as is represented in 

 the upper figure of Plate I., we introduce a lamina of sulphate 

 of lime below the crystals, the appearance changes to that re- 

 presented in the lower figure. 



If we turn the polarizer 90° round, the red crystals become 

 green, and vice versa. 



It is to be observed, that the otitlines only are coloured ; the 

 central parts of the crystals are generally white. 



This coloration of the outlines constitutes the great beauty 

 of some of these experiments. Its delicacy however is such 

 as scarcely to admit of being successfully represented. 



The upper figure in Plate II. represents another specimen 

 of ihe same crystal, different in shape only. If now we ex- 

 change the lamina of sulphate of lime for one of different 

 thickness, the colours change, as shown in the lower figure of 

 the same plate. 



Besides the boracic acid, I find the nitrates of potash and 

 soda have this property in a high degree. No doubt the list 

 may be considerably augmented ; yet I think that the crystals 



