32 THE INTERFEROMETRY OF 



pencils b and &', figure 17, to the principal plane pp, is always the same; but 

 pencils different in lateral position are successively selected. On the other 

 hand, when the prism P' is moved in the direction y, parallel to bb', path- 

 difference only is introduced, while the pencils selected remain the same. 

 Supposing the ordinary conditions of visibility (magnification, etc.) to remain 

 unaltered throughout, the wave-fronts are, as it were, explored in depth as 

 to their uniformity i.e., the distance is apparently recorded throughout 

 which a wave-train consists of identical wave-elements. Effectively, however, 

 the rapidity with which fringes decrease in size beyond visibility is directly 

 in question. Finally, when the opaque mirror M (or AT) is moved from M 

 to M', both effects occur together. Path-difference x = 2e cos 5/2 is introduced 

 and the pencil is displaced from b to b' . 



The ^-effect is thus probably the same as if P' were displaced to the left 

 and 5 were brought forward. Hence it is of great interest to determine the 

 extent in which the values of y and x are different. It appears as if the dis- 

 tance within which the wave-trains are uniform is definitely limited and that 

 it increases with the breadth of effective wave-front just instanced, while 

 both increase with the amount of dispersion to which the incident white 

 pencil has been subjected. Diffraction at the slit of the collimator may be 

 regarded as the first dispersion. This seems to me to be a very important 

 observation, and a systematic investigation of the lengths of uniform wave- 

 trains, so understood, in their dependence on dispersion, is desirable, even if 

 the geometry of the system should prove to be adequate to explain the 

 phenomena. 



17. Film grating. The method of two gratings was now again resorted to, 

 except that the first at G, figure 3 , was a film grating. This attempt failed 

 in my earlier work, when but a single film grating was used for the two dif- 

 fractions, because of insufficient light.* In the present case, where two 

 gratings (G 1 being reflecting) are employed, the method succeeded at once. 

 The first grating constant was Z7 = io" G Xi67 cm. ; observations were therefore 

 necessarily made in the second order of G', so that the spectra are not as 

 intense as with prisms. But the fringes are perfect and may be made as large 

 as desirable with but two in the breadth of the spectrum, for instance. They 

 come in and go out of range with inflation of form, and they are free from the 

 awns seen in the preceding paragraph (with prism), probably because the 

 light is less intense. 



The phenomena in general are the same in character; but the range of 

 displacement of either mirror is enhanced, conformably with the increased 

 dispersion at G. This range was found to be about 6 mm. under the best 

 conditions (arrows). If both M and N are successively displaced in the 

 same direction, the total displacement available between the hair-like fringes 

 at the extremes is about 1.5 cm. for each mirror. 



* I have since obtained the fringes with a single film grating. 



