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Connection of Chemical Forces with Polarization of Light 65 



lit as it would be impossible to explain the nature of his 

 investigations, or their results, without a preh'minary knowledge 

 of the meaning of the terms, "circular polarization," and *^hemi- 

 hedrism," it was necessary first to enter a little upon the expla- 

 nation of them. 



Accordingly a ray was explained as being a direction of light, 

 having no relations to space which differed from each other in 

 directions perpendicular to its length. Thus without complica- 

 tmg the subject, by using the language of the beautiful wave- 

 theory, a ray might be imagined as a cylinder of minutest diam- 

 eter but indefinite length.' When such a ray is reflected at a cer- 

 tain angle from glass or such like substance, it is split into two ; 

 one going into, and through the glass if it be not opaque, the 

 other being reflected from it. These two rays no longer possess 

 the same *' absence of sides" as the original ray. For the one 

 has been as it were flattened down to a "strip," while the other 

 has also been flattened similarly into a "strip," but the latter 

 strip is at right angles in regard to its "flattened plane" to what 

 the other is. A similar bifurcation of the ray is produced in the 

 interior of what are called doubly refracting crystals- This bifur- 

 cation and flattening of the ray is termed "plane polarization" of 

 1^ It; and it is so far a true instance of polarity — as that the two 



rays nave equal and sunilar properties m opposed directions. 



This was exhibited by the lime-light. The double image of a 

 small round hole formed by a crystal of Iceland spar was thrown 

 on a screen, and each beam shown to be most capable of reflection 

 in a plane in which the other was incapable of being reflected at 

 all. The action of the tourmaline as a doubly refracting crystal 

 which absorbs one of the rays was then explained ; and it was 

 shown that the position of the tourmaline in wliich it intercepted 

 one ray entirely, was exactly the position in which it gave the 

 other ray free passage. The optic axis of a crystal was then de- 

 fined to be a direction in if along which the light could pass 

 through the crystal without undergoing any chatjge whatever. 

 / The central ray of a polarized beam of light, traversing a piece 



of calc-spar along its optic axis, was shown to be intercepted or 

 transmitted by a tourmaline, precisely as if the section of the 

 crystal of calc-spar were away. 



An exception was however stated to exist to this law of the 

 neutrality of the optic axis. When a section of quartz, cut so 

 that the beam could career along its optic axis, was put in the 

 F^th of the polarized ray, it was found that instead of |)ermittiug 

 the ray to be eclipsed by the tourmaline when this was placed 

 in the position to eclipse it, that ray on the contrary fell on the 

 screen endowed with beautiful color; and furthermore that the 

 revolution of the tourmaline induced the most brilliant succes- 



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Second Skiii«8. YoL XII, No. 34.— July, 185L ^ * 9 



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