386 



SCIENCE 



[N. S. Vol. XLIX. No. 1268 



ploy strictly accurate surfaces; so that tlie 

 prisms with which grandfather used to dec- 

 orate his gas fixtures will, as a rule, suffice 

 admirably. In the figure P is such a prism 

 (truncated) on a plate of obsidian Q, the 

 long edges being normal to a white window 

 curtain at L near by, illuminated with sun 

 light or day light; or any light toward the 

 front, overhead, is good. 



The rays that are wanted, s, will enter 

 symmetrically at a mean angle of about 30° to 

 the vertical and after reflection at the base 

 of the prism and the plate, reach the eye in the 

 directionJS'. The rays totally reflected, t, come 

 from a greater angle to the vertical and are 

 not wanted. 



The limit of total reflection here (also 

 easily recognized) is usually a sharp parabolic 

 or cuspidal apex. The light seen through 

 either face enters by the opposed face. On 

 looking down from a steeper angle and with 

 properly selected faces, brilliant groups of 

 complete confocal ellipses (major axis one half 

 to over two inches), of confocal hyperbolae may 

 be seen in each of the roof faces. To find ad- 

 vantageous combinations, the three faces of 

 each prism should be examined in succession, 

 and it is well to rub P on Q to improve the 

 contact. On moving the eye fore and aft or 

 using different pressures, any type of ellipse 

 with white or colored disc may be produced at 

 pleasure. It is usually preferable to use a 

 shorter plate Q than is given in the figure, 

 about one half the length of the prism. 



When well produced the ellipses may also be 



seen by side light, with different patterns in 

 the two roof -faces. 



The type of interference figure clearly de- 

 pends on micromotric differences of the faces 

 in contact. The ellipses are Newton's rings 

 modified by the color dispersion of the glass. 

 The hyperbolae, however, are about equally 

 frequent; but their character is less easily 

 stated. They probably originate in cylindrics. 

 The ease of the 45°— 90° prism, with the right 

 angled faces respectively horizontal (on the 

 plate) and vertical, is also interesting; for 

 here the ellipses are apt to be circles with each 

 of the two groups seen after two reflections, 

 one in each of the orthogonal faces. The 

 light should enter nearly normal to the ob- 

 lique face. As it leaves in the same way, 

 one should observe through a horizontal slot 

 in a white screen. 



I may add a similar observation : If a cylin- 

 drical lens (say 1 diopter) is placed on a plate 

 and illuminated with homogeneous light, the 

 interference pattern consists of a succession 

 of equidistant arrow heads ■ along the line of 

 contact, all pointing in its direction. Now 

 these are the very forms observed in the inter- 

 ferences of reversed spectra along the line of 

 coincidence of spectra, except that the latter 

 are apt to be far narrower than the former. 

 It seems therefore, as if the effect of color va- 

 riation in one case and of the cylindric in- 

 crease of thickness of air fihn, in the other, 

 were formally capable of like treatment. 



Carl Bakus 



Brown Universitt, 



PROVniENCE, R. I. 



SCIENCE 



A Weekly Journal devoted to the Advancement of 

 Science, publishing the official notices and pro- 

 ceedings of the American Association for 

 the Advancement of Science 



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