The London, Edinburgh and Dublin philosophical magazine and journal of science . he observer to the middle of his field of view. Drawa plane perpendicular to the optic axis, preferably near tothe object looked at. This we may call the base-plane. Withc (the intersection of the optic axis and base-plane) as centre,describe a hemisphere with its convex side towards theobserver, and with radii of any assumed length, R. This wemay call the reference-hemisphere. 8. In doubly-refracting media we have to use half thewaA^e-surface of the medium instead of the simple hemispherewhich suffices for isotro
The London, Edinburgh and Dublin philosophical magazine and journal of science . he observer to the middle of his field of view. Drawa plane perpendicular to the optic axis, preferably near tothe object looked at. This we may call the base-plane. Withc (the intersection of the optic axis and base-plane) as centre,describe a hemisphere with its convex side towards theobserver, and with radii of any assumed length, R. This wemay call the reference-hemisphere. 8. In doubly-refracting media we have to use half thewaA^e-surface of the medium instead of the simple hemispherewhich suffices for isotropic media. But for the present weintend to confine our attention to the latter. Now (see Theorem 1. on p. 573 of the B. A. Report tov1901, or in § 5 above) the light with which we are dealing-whatever it is, might be withdrawn, and undulations of flat 2(38 Dr. G. J. Stoney on the Resolution of Light wavelets substituted for it, without producing any change innature. Each one of these u f ws (undulations of flatwavelets) travels in the direction of some radius of the Fis. 1 ^. J??a??e Reference-hemisphere. reference-hemisphere, and has its wave-fronts parallel tothe tangent-plane at the outer end of that radius We maycall undulations outward-bound when they travel along theradius from centre to surface of the hemisphere, and inward-ly ouad when they travel in the opposite direction. 9. Practically, in most real problems, we know beforehandwhether we are dealing with outward-bound or inward-boundundulations, so that no appreciable inconvenience resultsfrom the circumstance that two undulations—one inward-bound and the other outward-bound—are represented by thesame radius cPi- We may call cFl the guide-radius ofwhichever of the two we have to deal with. It may equallywell be represented by the point P1 on the hemisphere (whichwe may call its guide-point) ; and still better by Ii, theorthogonal projection of P1 on either the base-plane or someparallel plane (which we may call
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