Double refraction shape

The turbid liquids which were thus obtained were found to possess not only the usual properties of liquids (such as the property of flowing and of assuming a perfectly spherical shape when suspended in a liquid of the same density), but also those properties which had hitherto been observed only in the case of solid crystalline substances, viz. the property of double refraction and of giving interference colours when examined by polarised light the turbid liquids are anisotropic. To such liquids, the optical properties of which were discovered by 0. Lehmann, the name liquid crystals, or crystalline liquids, was given." Since the term crystal implies the existence of a definite space lattice, which is not found in the case of liquid crystals, it is perhaps better to use the term anisotropic liquids, ... 

Under all normal conditions the sign of the first kind of double refraction in collagen is positive relative to an optic axis paralleling the fiber axis. According to optical theory of colloidal systems (79, 188) this means that the macroscopic collagen fiber contains submicroscopic particles of anisodiametric shape with long dimensions aligned with the fiber axis. 

Kuhn (129) considered gelatin molecules in warm aqueous solutions to be randomly contracted to near spherical shape, and to yield weak streaming double refraction because of stretching under the shearing... 

However, when applying results of the type (57), it must be borne in mind that such equations apply to the amorphous constituent only, and must fail at advanced degrees of stretch where crystalliisation sets in since the influence of crystallisation on the double refraction of rubber-like polymers is considerable In addition to the intrinsic double refraction of the gel frame, the swollen gel may show structural double refraction and adsorption double refraction. The structural, or textural , birefringence is caused by the difference between the refractive indices of polymer substance and solvent. It was estimated by Wiener for rod-shaped particles and for platelets. Wiener s theory is not applicable to rods of molecular thickness, and the structual double refraction caused by oriented long-chain molecules has not yet been accessible to a reliable theoretical treatment. 

A negative double refraction was observed. This observation did not agree with what Thiessen and Triebel expected. They were of the opinion that rod-shaped particles occur in soap solutions. In that case a positive double refraction would be the result. It fits into their line of thought that the particles have a negative double refraction of their own (according to them the T( mpX particles are crystalline). It must however be remembered that this investigation can just be evidence that the micelles are platelike (see the work of Hess, p. 694). In this latter case a negative double refraction would have to occur. 

Microfocusing devices that are based on refraction operate in the same way as visible light optics but there are some differences (Schroer et al. 2005). Firstly, the X-ray refractive index of a material is smaller than in vacuum or air and, therefore, an X-ray focusing lens has a double concave shape. Secondly, because the refractive index of all materials is very close to unity for hard X-rays, the deflection is usually very small and many lenses have to be placed in series to achieve reasonably short focal lengths. In order to keep absorption to a minimum, these compound refractive lenses (CRTs) should be made from low-Z materials such as beryUium, carbon, aluminum, and silicon. CRTs with parabolic shapes made from polycrystalline aluminum by a pressing technique have proven to be well suited for microanalysis and full field microscopy applications for 20-120 keV X-rays (Lengerer et al. 1998). 

Double refraction of the various crystal forms was unaltered after staining X-ray-diffraction (X.R.D ) analysis revealed that the single sword-shaped crystal was tri—oxy-purine di-hydrate with and without methylene blue. The murexide test could be carried out even with stained crystals. 

Hatschek (1921) found that when a 10% gel was deformed and maintained in a state of constant strain for five days, the stress relaxed, so that the sample kept its deformed shape even after release of the load (see Section a, page 31). However, the strain double refraction remained. This result can also be explained on the basis of orientation of structural elements, as will be discussed in more detail in Section 4, page 44. 

From studies of double refraction of flow (Edsall, Foster, and Schein-berg, 1947), and sedimentation and viscosity (Oncley, Scatchard, and Brown, 1947), the dimensions of the fibrinogen molecule considered as an elongated ellipsoid of revolution have been estimated to be 35 A. X 700 A., with a molecular weight of about 500,000 assumption of the shape of a cylindrical rod would probably not alter these figures greatly. 

The fibres are double-refracting because of their two-phase nature. This is known as form birefringence and arises because of the differing refractive indices of pores (1.00) and alumina (n 1.56 a 1.75) and because of the nonspherical shape of sate of the pores (ref. 4). 

Now as regards the nomenclature one has so far called the double refraction appearing after a particular action on a system accidental double refraction thereby thus making a distinction with the double refraction already present in a given system, which is called proper double refraction, except in particular cases (only occurring in colloid systems) to which we shall return presently and which were included in the concept shape double refraction. 

Some liquid colloid systems show a double refraction when they are set in laminar flow. The classical example is the V2O5 sol which (at least after ageing) consists of needle-shaped particles with a crystalline character the sol itself with its particles randomly oriented by Brownian motion is isotropic if now one makes it flow through a tube or between two coaxial cylinders, one of which is rotating, it becomes double refracting The same is the case with many macromolecular sols This phenomenon, streaming double refraction of sols, is also reckoned as accidental double refraction, because it only occurs through a constraint exerted on the system ... 

Tile nomenclature depicted, which is based on phenomenology, leads to various difiScuIries, if one takes account of the nature of the phenomena The double refraction of a cellulose membrane, obtained from nature, for example, would be called proper double refraction, that of an artificial cellulose membrane, which one has obtained by stretching an isotropic cellulose gel, would on the contrary have tP be reckoned as accidental double refraction, while in both cases one is dealing with the same phenomenon Furthermore the nomenclature is not entirely consequential even from a phenomenological point of view, since the shape double refraction in many cases arises through an external intervention and then would therefore have actually to be reckoned as accidental ... 

With perfectly elastic systems, such as, for example, vulcanised rubber, which return to their original shape after the removal of the applied strains, the double refraction can sometimes also disappear completely after the removal of the load. It serves no purpose however to introduce a separate name for such reversible " cases Finally we must still discuss a special form of double refraction, which occurs exclusively in colloid systems. A system consisting of parallel oriented needle-shaped isotropic particles with refractive index embedded in a medium with refractive index nm, exhibits the double refraction of an optically uniaxial crystal ... 

The double refraction (that is — n, ) is plotted as ordinate, the refractive index of the various liquids as abscissa. Fig. 27 holds correspondingly for discs. The variation of A with thus shows whether one is dealing with shape double refraction of isotropic particles and if so v hethcr rod or disc double refraction plays a part. 

The Wiener double refraction is indicated by the name shape double refraction or also according to the structure of the system by rod- or disc double refraction. Since however we arc here always concerned with oriented structures, we must include it as a special case in the classification orientation double refraction ... 

H the rods or plates do not — as assumed in the theory — lie exactly parallel, but are imperfectly oriented, the" system in question will still be able to exhibit a shape double refraction component We shall not however go into this as no theoretical treatment of these cases is known in the literature ... 

A further description of this phenomenon is to be found in chap III, 3 b, p 109 and in vol II, chap IV, 7, p 115 chap V, 7, p 142 chap XIV, 3, p 693, 6 p 719 Magnetic and electric doul le refraction in liquid systems is also almost always an orientation double refraction See chap III, 3 c, p 113 If it concerns an orientation of molecules, then one has only to do with the proper double refraction of these molecules with particles of colloidal dimensions there is in addition a second component the shape double refraction ... 

It can be understood that it is a great simplification for the development of the theory that the axes of the intrinsic anisotropy as a rule coincide with those of the shape anisotropy. The experimental set-up is simpler than with streaming double refraction since the orientation is produced in this case by bringing the solution into an electric or a magnetic field. 

When a slight amount of salt is present, it forms a colloidal solution, containing rod-shaped particles. The solution shows streaming double refraction. Concentrated solutions may gel and a paste of the dye with water is typically plastic. 

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