On the theory of certain bands seen in the spectrum / by G.G. Stokes.

  • Sir George Stokes, 1st Baronet
Date:
1848
Catalogue details

Licence: Public Domain Mark

Credit: On the theory of certain bands seen in the spectrum / by G.G. Stokes. Source: Wellcome Collection.

    7/18 page 231
    or negative, gives the number of bands seen between F and G. For anything that appears from the imperfect theory of the bands given in the preceding article, it would seem that the sign of N was of no consequence. It will presently be seen, however, that the sign is of great importance : it will be found in fact that the sign + indicates that the second arrangement mentioned in art. 2 must be employed ; that is to say, the plate must be made to intercept light from the thin end of the prism, while the sign — indicates'that the first arrangement is required. It is hardly necessary to remark that, if N should be fractional, we must, instead of the number of bands, speak of the number of band-intervals and the fraction of an interval. Although the number of bands depends on nothing but the values of N, the values of M are not without physical interest. For M expresses, as we have seen, tiie number of waves’ lengths whereby one of the interfering streams is before or behind the other. Mr. Airy speaks of the formation of rings with the light of a spirit-lamp when the retardation of one of the interfering streams is as much as fifty or sixty waves’ lengths. But in some of Professor Powell’s experiments, bands were seen which must have been produced by retardations of several hundred waves’ lengths. This exalts our ideas of the regularity which must be attributed to the undulations. 6. It appears then that the calculation of the number of bands is reduced to that of the retardation R. As the calculation of R is frequently required in physical optics, it will not be necessary to enter into much detail on this point. Tiie mode of performing the calculation, according to the circumstances of the experiment, will best be explained by a few examples. Suppose the retarding plate to belong to an ordinary medium, and to be placed so as to intercept light from the thin end of the prism, and to have its plane equally inclined to the faces of the prism. Suppose the prism turned till one of the fixed lines, as F, is seen at a minimum deviation; then the colours about F are incident perpendicularly on the plate ; and all the colours may without material error be sup- posed to be incident perpendicularly, since the directions of the different colours are only separated by the dispersion accompanying the first refraction into the fluid, and near the normal a small change in the angle of incidence produces only a very small change in the retardation. The dispersion accompanying the first refraction into the fluid has been spoken of as if the light were refracted from air directly into the fluid, which is allowable, since the glass sides of the hollow prism, being bounded by parallel surfaces, may be dispensed with in the explanation. Let T be the thick- ness of the plate, /!* the refractive index of the fluid, that of the plate ; then R=(^'-^.)T (2.) If the plate had been placed so as to intercept light from the thick end of the prism, we should have had — R=(|M/' —|«/)T, which would have agreed with (2.) if we had supposed T negative. For the future T will be reckoned positive when the plate intercepts light from the thin end of the prism, and negative when it intercepts light
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