282 



SCIENCE 



[N. S. Vol. XLin. No. 1104 



and Horsburgh's Modern Instruments and 

 Methods of Calculation, by C. C. Grove; 

 Longley's Tables and Formulas, revised edi- 

 tion, by Joseph. Lipka; Enriques' Vorlesucngen 

 iiber projektive Geometrie, second German edi- 

 tion, by A. Emch; Miller's Descriptive Geom- 

 etry, Armstrong's Descriptive Geometry, and 

 Grossmann's Darstellende Geometrie, by Virgil 

 Snyder ; " ISTotes ; " and " ISTew Publications." 



SPECIAL ARTICLES 



AN APPARENT LATERAL REACTION BETWEEN 



IDENTICAL PENCILS OF LIGHT WAVES, 



CROSSING EACH OTHER AT A 



SMALL ANGLE! 



1. Methods. — To exchange the component 

 beams in the interferometer, to mutually re- 

 place the two pencils which interfere, is not 

 an unusual desideratum. To replace two 

 pencils of component rays travelling more or 

 less parallel to each other, by pencils more or 

 less normal to each other, to be able to operate 



pencils are diffracted along the same direction 

 G'T, into the telescope at T. 



If now the opaque mirrors m, n, M, N, are 

 appropriately rotated, the parallel component 

 beams GmMG' and GnNG' may be replaced by 

 GmNG' and GnMG', respectively, which crosa 

 each other at c, while the pencils impinging at 

 G' have been exchanged. 



There is an essential difference in these two 

 cases. Whereas in the case of parallel rays, 

 a', and 6', the double diffraction is an incre- 

 ment of either, in the case of the crossed 

 rays, a and h, it is a decrement and the system 

 tends to become achromatic. In the latter case 

 one shoiild suppose that homogeneous light 

 and a wide slit only could be used in the inter- 

 ferometer. But this is not so. 



2. Results. — The reflecting gratings with 

 large dispersion constants in my possession 

 waste too much light and the work is thus 

 biu-densome. The following results were 



I'll 



at the point of intersection of corresponding 

 pencils of rays from the same source, crossing 

 at any angle, may be of interest in a variety 

 of operations and may even suggest novel ex- 

 periments. 



In Pig. 1, I have sketched one of many 

 forms of apparatus of the kind in question, 

 with which I have recently been working. A 

 beam of parallel rays from a collimator, L, 

 impinges on the reflecting plate grating G. 

 The diffracted pencils a, i, are reflected by the 

 opaque mirrors n and m into V and a', to be 

 again reflected by the opaque mirrors M and 

 N into the pencils h" and a". These impinge 

 on the plate grating G', so placed that both 



1 Work done on a grant from the Carnegie In- 

 stitution of Washington, D. C. 



therefore investigated with a good ruled trans- 

 mitting grating, adjusted to secure the double 

 diffraction of Fig. 1 in a single grating. This 

 simplifies the method and the interferences 

 are much more expeditiously found. The rays 

 in such an apparatus must cross in the glass 

 plate of the grating at c. 



In the case of parallel rays Nn, Mm, white 

 light and a fine slit, I obtained the linear phe- 

 nomena of reversed spectra as usual. On using 

 homogeneous light and a wide slit superb in- 

 terferometer fringes were obtained. In every 

 instance these are parallel striations crossing 

 the whole field uniformly. They may easily be 

 made coarser or finer, or rotated at pleasure, 

 but a given field never shows independent 

 groups; i. e., there is no second periodicity 



