BAIRD &. TATLOCK (LONDON) LTD. 





Polariscopes, by Schmidt & Haensch. 



Agents : BAIRD & TATLOCK (LONDON) LTD. 



DESCRIPTION OF THE POLARISCOPE. 



In an ordinary ray of light the vibrations of the particles of ether take place simultaneously in all directions perpendicular 

 to its axis. By certain means it is possible to restrict the vibration to any one particular direction. It is then called a linear 



This conversion of ordinary into polarised light may be brought about first by reflection, which need hardly be considered 

 here, and secondly by repeated single refractions or by double refraction in certain crystals notably calc spar. 



In a natural crystal of Iceland spar (Fig. i) the principal axis lies in the line joining the points d and /, where three obtuse 

 angles meet. Suppose a plane to pass through the shorter diagonals of two opposite faces of the rhomb, i.e., either d g, a f, or 

 d b, h f, or d e, c /, it will always contain the principal axis d f. Any such plane, and all planes parallel thereto, are called principal 

 sections of the prism. If a ray of light m n (Fig. 2) falls on one face, as a b dc (of which d b h f represents the principal section), 

 it will, on entering the crystal, divide into two refracted rays unequally bent. Both are polarised, and it will be found that the 

 plane of polarisation (or plane of vibration) of the less refracted or extraordinary ray n q is perpendicular to the principal section 

 d b h /, while that of the more highly refracted or ordinary ray n p coincides with the plane of the section. 



For polariscopic purposes it is best to give exit to one of the two polarised rays, namely, that one in the direction of the 

 incident light, and to eliminate the other ray. This can be done by converting the calc spar into a Nicol prism. To this end 

 a piece of calc spar is split into an elongated rhombohedron, as in Fig. 3, in which the plane passing through the points abed 

 represents a principal section. 



The natural ends of the prism a f b e and d g c h, the former of which is inclined to a d and the latter to c b at an angle of 

 1 7 degrees, are ground so as to reduce their angles to 68 degrees (see Fig. 4). The prism is then divided in the direction b' d', which 

 is perpendicular to a b' and c d', and the halves after polishing the faces of the section are united with Canada balsam. The 

 sides are then blackened, and the Nicol is fixed with cork or other suitable material into a brass case. The principal section of 

 the prism passes through the shorter diagonals of the two rhombic ends. If a ray of light / m (Fig. 5) parallel to the edges of 

 the longer side falls on the face a b', it is divided into rwo rays. The less refracted (or extraordinary) ray m p q traverses the 

 film of balsam at p, and emerges in the direction q s parallel to I m. The more refracted (or ordinary) ray m o meets the balsam 

 at o, which, from its being a medium of so much feebler refractive power, causes total reflection of the ray in the direction o r, 

 whereby it becomes absorbed by the case of the prism. The other ray emerges in the direction of the incident ray, but possesses 

 only half of its luminous power. The plane of polarisation (and vibration) of this ray is at right angles to the principal section, 

 and therefore passes through the longer diagonals of the end faces of the prism. 



Suppose now two Nicol prisms be taken, the one fixed and directed towards a source of light, the other so arranged that it 

 has a movement of rotation about its longitudinal axis. The first prism may be called the polariser ; the second, the analyser. 

 Between the Nicols is placed an empty tube or one filled with water. On looking through the system while revolving the analyser 

 a position will be found in which light no longer passes through. On continuing the rotation of the analyser through half a com- 

 plete revolution another maximum of darkness will be found. For purposes of observation, the points of greatest darkness are 

 to be preferred for reference, since at these points the least movement of the Nicol produces a perceptible change in the appearance 

 of the field of vision. 



If now the tube be filled with a solution of some substance capable of rotating polarised light such as cane sugar, the analyser 

 having been previously set to darkness, it will be found that the field of vision appears bright, and to obtain the maximum of 

 darkness again the analyser must be turned to the right through a certain angle. Since no rays could pass through when the 

 plane of polarisation of the analyser was at right angles to that of the polariser, it must be concluded that after the introduction 

 of the tube containing the sugar solution the rays have experienced a certain deflection of their plane of vibration. 



The angle through which the analyser has to be turned to bring about a recurrence of darkness is termed the angle o/ rotation, 

 and is the measure of the deflection experienced by the plane of polarisation. 



The rotation of the plane of polarisation is called circular polarisation. Substances which exhibit this phenomenon are said 

 to be circular polarising or optically active, and are distinguished as dextro-rotary or laevo-rotary in accordance as to whether 

 they turn the plane of polarised light to the right or left, whilst those substances which do not possess this power are said 

 to be optically inactive. 



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