On the connexion of chemical forces with the polarization of light / Nevil Story Maskelyne.

  • Nevil Story Maskelyne
Date:
1851
Catalogue details

Licence: Public Domain Mark

Credit: On the connexion of chemical forces with the polarization of light / Nevil Story Maskelyne. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    M. Biot has been for forty years enriching cliemico-physical science by a series of memoirs detailing the results of his study of these phenomena. He has there shown the value of this means of tracing changes in chemico-molecular constitution. M. Pasteur has carried forward this inquiry into a new channel by tracing a connexion between this property in substances, of circularly polarizing light, and their crystalline character. But as it would be impossible to explain the nature of his investi- gations, or their results, without a preliminary knowledge of the meaning of the terms “ circular polarization,’’ and “ hemihedrism,” it was necessary first to enter a little upon the explanation 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 complicating the subject, by using the language of the beautiful wave-theory, a ray might be imagined as a cylinder of minutest diameter 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 flat- tened 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 bifurcation and flattening of the ray is termed “ plane polarization ” of it; and it is so far a true instance of polarity—as that the two rays have equal and similar properties in 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 posi- tion of the tourmaline in which 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 defined to be a direction in it along which the light could pass through the crystal without under- going any change 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 path of the polarized ray, it was found that instead of permitting the ray to be eclipsed by the tourmaline when this was placed in the position