76 



THE INTERFEROMETRY OF 



with the principal focus at F, to be observed with the eye-piece. The screen 

 5 s f with a double slit parallel to the linear source L is placed in front of the 

 objective. Brilliant interferences are then seen at F, which are coarser as 

 the distance e between the slits s s' is smaller, and if the distance PF is r and 

 the distance between fringes is x, the usual equation 



\/e=x/r 



is applicable. Owing to the direct application of this experiment to the above 

 investigations, its very fundamental importance in the theory of resolution 

 of optical instruments, etc., it seemed worth while to give it experimental 

 treatment here. The screen 5 5' is conveniently made by cutting parallel 

 lines with a sharp triangular cutting stylus and a steel T-square, on a black- 

 ened gelatine dry plate. A number of such doublets 0.05 to 0.5 cm. apart 

 may be ruled at a distance of about 0.5 inch apart. When the parts are as- 

 sembled, the fringes may be seen in all focal planes F, and the fringes are in 

 fact much more brilliant with the ocular out of focus. The enormously large 

 diffraction of each single slit is simultaneously visible, and if two doublets 

 are close enough together, their systems may be seen superposed. If c is small 

 (less than o.i cm.) the fringes are in fact visible to the naked eye without a 

 telescope. With two identical doublets close together, the fringes may be 

 seen to be alternately in step and out, as the ocular of the telescope moves 

 outward, until finally the diffractions of the "rod" between the doublets is 

 strikingly manifest. This has also been generalized in the present work. 



In all these classical cases there is a continuous succession of pairs of corre~ 

 spending points (one of the pair in each slit of the doublet) between which 

 interferences occur. The line of any two such points is rigidly normal to the 

 direction of the slits. In the above experiments with spectra, however, the 

 two points may not only have any relation to each other, but either point 

 may be moved at pleasure. This gives rise to the bewildering variety of 

 beautiful phenomena, some of them useful, which I have tried to describe 

 in the present and preceding reports. 



With the beams of like origin separated, it is next necessary to bring them 

 together again. This requires at least one independently controllable reflec- 

 tion for each beam. In the interesting group of phenomena obtained with 

 crossed rays, two reflections may be desirable, though with a change of appa- 

 ratus a single reflection here also suffices. Thereafter the beams may be com- 

 pounded in a manner inverse to the one by which they were produced, for 

 instance, by the reflection of a silvered obtuse prism, by refraction toward 

 the edge of a prism, by a grating, by polarization, etc. To observe the recom- 



