106 REVERSED AND NON-REVERSED SPECTRA. 



If = go, then sin '= (1/2) (\/2/x 2 i i). Thus if n= i. 55, then sin ^ = 0.47 5, 

 and ' = 28.4. Now, since i+5= 6, the angle 6 will obviously have to be in 

 the second order of the spectra of the grating G. 



Although the two spectra obtained in this way were highly dispersed and 

 very brilliant, the interference phenomenon itself was not much superior to 

 the case where reflection from the (silvered) faces of the prism was employed. 

 The fringes disappeared, in fact, for a displacement of i or 2 mm. of the mirror 

 M, showing the usual inflation of form just before vanishing. The details 

 also were of the same nature, the large arrow-shaped forms being obtained 

 when illuminated strips on the grating were superposed and the latter slightly 

 rotated until the maximal conditions appeared. 



To increase the range, the angle 5 must be reduced, as far as practicable. 

 This is possible in the present method, since the points of intersection at a 

 and G may be made to all but coincide. Reflection from the mirrors M and 

 N would then be normal. To attain this end it will be necessary either to 

 have the grating constant or the prism angle $ predetermined, or to use rays 

 of suitable divergence at L. 



56. Methods with paired prisms. White light (fig. 76, L) from a collimator 

 is reflected in turn from the silvered sides of the sharp prism P, from the 

 opaque mirrors M and N, and from the silvered blunt prism P', as shown by 

 the component beams abc and a'b'c'. Thereafter the white beams are diffracted 

 by an Ives film grating G, with attached prism p, and observed in a telescope 

 at 7\ Interference, therefore, takes place in the focal plane of the telescope 

 and would not (for the case in fig. 76) occur in its absence. Very interesting 

 results were obtained with this apparatus. The spectra are non-reversed or 

 else (if slit and grating are rotated 90) inverted. The work, however, is 

 still in progress and will be described elsewhere. I will merely add, in this 

 place, that the work with prisms is important, inasmuch as it shows the essen- 

 tial part played by the diffraction of the slit of the collimator, in its bearing 

 on the phenomena of the present report. It is the function of the prism P to 

 cleave the diffracted field which leaves the collimator. For this reason pencils 

 identical in source are found on both sides of P. The experiments thus fur- 

 nish the final link in the theory of the phenomena. 



Furthermore, as the above results already show, the range of displace- 

 ment of either opaque mirror (M, N) within which interference fringes are 

 visible, increases in marked degree with the dispersion to which the white ray 

 is subjected on separation and before the resulting partial rays reach their 

 final recombination. These ranges increase from a fraction of a millimeter 

 to almost a centimeter, while the width of the strip of spectrum carrying 

 the interference fringes, caet. par., remains the same. This also has a fun- 

 damental bearing on the phenomenon and is under investigation. The ques- 

 tion at issue is whether increase of range of displacement results simply 

 from the geometry of the optic system, or whether wave-trains are actually 

 uniform throughout greater lengths, in proportion as they have been more 

 highly dispersed. 



