. The London, Edinburgh and Dublin philosophical magazine and journal of science. ized beam produced by this combination,whether right-handed or left-handed, has no special directionof polarization, aud it therefore presents the same aspect toall parts of the object under stress. It is again convertedto plane-polarized light by the inverse combination of quarterwave-plate and nicol and the interference fringes are stillproduced, while the black cross disappears. This arrange-ment has the further practical advantage that, except for aslight and invariable change of tint, the interference colour
. The London, Edinburgh and Dublin philosophical magazine and journal of science. ized beam produced by this combination,whether right-handed or left-handed, has no special directionof polarization, aud it therefore presents the same aspect toall parts of the object under stress. It is again convertedto plane-polarized light by the inverse combination of quarterwave-plate and nicol and the interference fringes are stillproduced, while the black cross disappears. This arrange-ment has the further practical advantage that, except for aslight and invariable change of tint, the interference coloursproduced are independent of the angular position of theobject. Fig. 6 shows the general arrangement of the colour fringespresented by the hook when viewed by circularly polarizedlight, and the stresses at points of the central section can bedetermined in a similar manner to that described above. 750 Prof. E. G. Coker on the The projected image of 0473 in. actual width measured2 ins. on the squared paper screen, and the stresses obtained are oiyen in the annexed table. Fig. LB - Light 8/?ow/vB = BLUE p z Purple Abscissae, inches. o .3 a a is pn 0. 2380337040604570 016 025. 05. Neutralaxis. 1-5. 2-0 1970 at 012380 at 0162480 at 0-25 1460 0-707070-75 95014601910 640127017202730 and a plot of these values is shown in fig. 7. The position of the neutral axis has been mathematicallydetermined by Andrews and Pearson*, and its distance y§from the centre of the section has been shown to be ^o = Po|l|(7i-772)^I-l}, * Drapers Company Research Memoirs. Technical Series. I. 1904. Optical Determination of Stress. 751 where p0 is the mean radius of curvature, c is the distance ofthe applied force from the centre of the section, and yu y2are constants depending on p0, 77, and T. In the present casery1= 1*048, 72 = 891, where 77 = *39, and the position of theneutral axis is 055 in. from the centre on the tension side. Fi-. 7.
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