REFRACTION.] 



UNDULATORY FOBCES. LIGHT. 



79 



subsisting in bodies, tends also to bring them into inti- 

 mate union ; and, as sucb, is the promoting cause of 

 phosphorescent effects. Heat, of course, produces the 

 appearance of fluorescence ; and, in that case, we there- 

 fore have one force producing another ; and a sufficient 

 cause is therefore existing towards the production of its 

 consequent effect. 



We shall now pass on to the subjects of Double Befrac- 

 tion and Polarisation, in which the reader will find that 

 all his previously acquired knowledge will be called into 

 requisition. These will be succeeded by articles on 

 Artificial Illumination and Photography two subjects 

 of the deepest interest as applications of the force or 

 agency of light. 



CHAPTER II. 

 DOUBLE REFRACTION AND POLARISATION OF LIGHT. 



WE have now to enter on a most interesting branch of 

 scientific inquiry, which, although of comparatively 

 recent origin, has attracted the profound attention of the 

 most eminent living pliilosophers. 



The reader will have already become acquainted with 

 many of the changes which a ray of light is subject to 

 when it is either reflected from, transmitted through, or 

 refracted by various substances ; and it will be essential 

 to our progress that he should bear in mind the various 

 laws on which we have already dilated. This will be 

 more fully seen when we have to deal with phenomena 

 which depend both on reflection and refraction, yet in 

 other conditions than those which we have previously 

 inquired into. 



In a former part of this section, we fully entered into 

 the consideration of the single refraction of light. We 

 observed, that when a ray of light passes from a 

 rare through a dense medium, and vice vend, it does 

 so in a line varying from that in which it had previously 

 travelled ; in other words, it suffers refraction : still, 

 hnwever, no visible change in its nature is apparent ; its 

 direction al >no being affected. There are, however, 

 a great variety of substances which have the power of 

 not only refracting light singly, but also of dividing 

 a ray into two parts an effect called dtmftU refraction. 

 The best known instance of this kind of bodies is the 

 cj.uinon Iceland spar. If a piece of it be placed on a 

 lin" itttuvn on jaiicr, or on a wafer, ifec., a double image 

 of the object will be perceived. The rays proceeding 

 from the object are divided into two pencils by the 

 peculiar structure of this doubly refracting body; and 

 BOOM the effect on the eye is to cause the perception of 

 a double image. To make this more easily understood, 

 we annex an engraving illustrating the case of double 

 refraction by a rhomb of Iceland spar, together with one 

 showing the direction of the rays of light passing through 

 a rhomb of that substance. 



Fig. 68. 



Fig. C8 represents a piece of Iceland spar, placed on 

 a paper on which the letters A B are printed. These, 

 when viewed through the spar, appear double, as illus- 

 trated in our diagram. Fig. 69 shows the division of 

 Fig. 89. the ray of light into 



two pencils, e o, when 

 passing through a 

 doubly refracting sub- 

 stance. The effect of 

 double refraction must 

 be owing to some pe- 

 culiar arrangement of 

 the particles of the re- 

 fracting body; or, in 

 other words, to its 

 molecular structure. 

 We find that a vast variety of bodies, which, in their 

 ordinary state, do not possess this property, may bo 



made doubly refracting by means of pressure, by 

 changes of their form by mechanical force, by suddenly 

 heating and cooling them, and through any cause which 

 alters the relative arrangement of their particles. Glass 

 may be taken as an illustration of this class of 

 stances ; for although but single refracting in its ordinary 

 state, it affords the phenomena of double refraction 

 on being suddenly heated or cooled, or when under 

 pressure or distortion. 



We have already stated that the phenomena of double 

 refraction is caused by a ray of light, on entering certain 

 bodies, being split into two rays or pencils, which ;nv 

 separated thereby at an angle from each other. In 

 bodies of a crystalline character, there are what are 

 called optic axes, or fixed lines ; and on the position and 

 nature of these, the direction, and consequent refraction, 

 of the ray of light depends. Some bodies have but one, 

 and others two axes of double refraction. As an 

 instance of a substance possessing one axis, we may 

 name Iceland spar ; and one possessing two axes we find 

 in crystallised nitre. These two classes of bodies are 

 respectively termed uniaxial and biaxial crystals. 



We must now explain more clearly what we mean 

 by an optic axis ; and for this purpose we refer to the 

 annexed engraving, which represents a rhomb of Icelau 

 spar. 



In Fig. 70, we have a re- 



Centation of a piece of Ice- 

 spar. If a line is sup- 

 posed to be drawn from a 6 

 to 6 e, this would represent 

 the axis of the mass. Let e 

 bo a ray of light falling per- 

 pendicularly on the surface 

 of the rhomb. Now if this 

 were of glass, the ray would, 

 according to the laws of sin- 

 gle refraction, pass through unchanged, because per- 

 pendicularly incident thereon.* But when falling on a 

 rhomb of Iceland spar, the ray is divided into two parts, 

 of which ef is the ordinary ray, and e y the extraordinary 

 ray. If the ray of incident light fall obliquely, still 

 the ordinary ray is obedient to the laws of single refrac- 

 tion, whilst the extraordinary pursues a course divergent 

 from the optic axis of the crystal, and at various angles 

 of position thereto, according to the substance under 

 examination. In observing the two images presented 

 by a piece of Iceland spar whilst it is turned round over 

 a spot, it will be found that the extraordinary ray moves 

 in a circle, whilst the ordinary remains stationary. The 

 relation of these two rays in reference to the optic axis 

 of the crystal, may thus be examined by the student, 

 and a knowledge of their mutual divergence and position 

 in various circumstances is at once obtained. The 

 optic axes are respectively positive and negative when 

 the extraordinary ray is bent either to or from the 

 geometrical axis of the crystal. As an instance of some 

 bodies possessing positive axes, we may mention quartz, 

 ice, <fec. ; whilst Iceland spar, the tourmaline, prussiate 

 of potass, <tc., have negative axes. We shall be able to 

 see the application of this law more fully when we enter 

 on the consideration of polarised light, and its resulting 

 phenomena of the production of colours, &c., under 

 certain circumstances. 



We must here state, that the velocities of the ordinary 



Sec ante, p. 45. 



