100 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



the centers are well indicated and the motion of rings adequately clear. With 

 the slit and tube, or slit tube and screen, the ellipses become sharp and the 

 fine lines indefinitely visible. The slit need not be very fine, but as it is finer 

 the velvety black lines on the colored spectrum become more marked. The 

 interference pattern is now quite as good as with the arc lamp. 



Naturally when the filament is so near the slit, the rays on leaving the 

 collimator diverge strongly in the vertical plane. Hence the illuminated parts 

 of the mirrors M and N may be 2 or 3 inches long. If these mirrors are of 

 ordinary plate glass it is not liable to be adequately perfect over the whole 

 length, and the ellipses will be imperfect in form. But this is not a serious 

 disadvantage. 



At wider ranges (g to M and N i to 2 meters), the arrangement is not very 

 satisfactory for photography, because the light passing through the telescope, 

 unless the objective is very large (a 24-inch objective was used), is only a 

 small part of that passing through the slit. Hence the light camera at the 

 end of the telescope is insufficiently illuminated. For photographic purposes 

 it would then seem to be better to place the Nernst filament at a distance 

 from the slit and to use a condenser ; but I was unable to obtain marked ad- 

 vantages in this way, while the condenser is an annoyance. Hence for photo- 

 graphic purposes it is better to replace the plane mirrors M and N by identical 

 concave mirrors in which the light is appropriately condensed. This is done 

 in the inclination apparatus in Chapter I, 21, and further reference has been 

 made there. In any case, however, greater steadiness and freedom from 

 tremor than the laboratory affords would be desirable for photography, and 

 though it is not difficult to obtain families of ellipses in the way given on the 

 ground-glass screen of the camera, few experiments in actual photography 

 were made. 



The spectrum of the Nernst filament is free from the Fraunhofer lines. It 

 is, however, easy to obtain the reversed D lines, by using an ordinary sodium 

 flame placed either in front of the slit or (contrary to expectations) even be- 

 hind it within the collimator. One would have expected the latter method 

 to interfere with the definition, but it does not seem to do so. When the 

 sodium lines have once been indicated, the cross-hairs of the telescope may 

 be placed in coincidence with them and the desirable fiducial lines of the spec- 

 trum thus obtained. 



PART IV SCATTERING IN THE CASE OF REGULAR REFLECTION FROM A 



TRANSPARENT GRATING, AN ANALOGY TO THE REFLECTION 



OF X-RAYS FROM CRYSTALS. 



64. The phenomenon. No doubt the following phenomenon has been 

 noticed before, but I have seen no description of it. If a vertical sheet of 

 white light L, from a collimator, is reflected from the two faces of a plate- 

 glass grating, having about 10,000 or more lines to the inch, g being the ruled 

 face, the two beams b and y going to the opaque mirror N are respectively 

 vividly blue and brownish yellow. In other words, more blue light is regu- 



