154 J. TJ. HAEFORD POLAKISATION OF LIGHT. 



phenomena in connexion 'with polarised light. First as regards 

 crystals. It is not to be concluded that Iceland spar is the only 

 substance possessing the property of double refraction. All crys- 

 talline forms, for the purpose of the present inquiry, may be 

 classified under three, or even two divisions. 1st, the cubic series, 

 including all forms that are based on or derived from the funda- 

 mental form of the cube. 2nd, the prismatic, which may be taken 

 to include the 3rd, the oblique. Now it is to be particularly noted 

 that the property of double refraction is possessed by the latter 

 only of these classes of forms, the cubic series of forms having no 

 such property. Then further note — first, that, in the cubic forms, 

 the axes of form, i.e. those lines which directly intersect the 

 crystal, are equal ; second, that experiment has shown that these 

 forms, when capable of expansion, are expansible or elastic equally 

 in all directions of their axes. On the contrary, the axes of the 

 oblique and prismatic forms are unequal, and experiment has 

 shown that their expansibility or elasticity is also unequal in. 

 different directions. It is fairly inferred from these considerations, 

 that in the structure (or architecture, as it has been called) of 

 crystals, the molecules of which they are built up have arranged 

 themselves in accordance with the properties above stated — viz. 

 that in the cubic crystals the density of the molecular arrangement 

 is equal in all dii-ections; or, in other words, that the attractive 

 force that holds them together is equal in all directions ; or again, 

 that the tension is similarly equal in all directions. Xow in the 

 case of the oblique and prismatic forms all the foregoing conditions 

 are different. They are found by experiment to be unequally 

 elastic and expansible. The density, attractive force, and tension, 

 therefore, in the arrangement of the molecules, are held to be 

 unequal in different directions. There is, in evident connexion 

 with these respective properties of crystals, the fact that a ray of 

 light suffers no separation in passing through a cubic form, but is 

 divided into two in passing through an oblique or prismatic form. 



The various substances or preparations used as illustrating under 

 the microscope some of the phenomena of polarisation may be 

 noticed here, as possessing the characteristics of unequal tension, 

 such as animal and vegetable tissue, bone, horn, fibres of silk, 

 wool and hair, seeds and seed vessels, and artificially prepared 

 bodies such as gum, jellies, resins, etc. Of the same character 

 may be instanced the optical property in respect to polarisation 

 exhibited by unannealed glass. A moulded form of glass is greatly 

 heated and then allowed to cool rapidly. The ettect is to contract 

 the outer surface in the act of cooling, in excess as compared with 

 the interior. Thus the molecular condition, or in other terms, the 

 tension, of the whole mass, is unequal. 



The consideration of the possible law which governs tlie arrange- 

 ment of the molecules of matter when they pass from the lluid to 

 the solid state in the act of crystallising, furnislu's the additional 

 support to tJH! use of (he term " polar " as :q)plic(l to light. It is 

 clear that the particles of matter arrange or build themselves up in 



