CHAPTER VIII. 



THE LINEAR TYPE OF DISPLACEMENT INTERFEROMETERS. 



57. Introductory. This apparatus will be referred to in various places in 

 this book and presents certain interesting features. The incidence of the 

 grating is normal (I = R = o), and both component rays in their vertical pro- 

 jection lie strictly in the same plane. To make the horizontal projection also 

 collinear is not quite possible in practice, because the direct or unreflected 

 rays and the corresponding spectra would overlap with the spectra of the 

 interferometer. As the former are much more intense, the interference pat- 

 terns would scarcely be visible in the combination. To avoid this, the rays 

 diverge slightly (a few degrees, depending on the distance between grating 

 and opaque mirrors) in a vertical plane. But this is of no consequence, as the 

 horizontal projections only are used in the measurements. One may note, 

 in passing, that this avoidance of coincidence with undesirable spectra secured 

 by tilting the grating and the corresponding opaque mirror in the same 

 direction is, in general, one of the essentials of the adjustments. 



The advantage of the linear displacement interferometer is this: that it 

 can be built on a rail and mounted along a wall or a pier. If the rail is tubular, 

 a current of water may be passed through it from the middle toward both 

 ends, to insure constancy of temperature. 



58. Apparatus. The apparatus was constructed as follows and gave good 

 results at once, showing strong interferences. The ellipses were, in fact, 

 oblate in the red, circular in the yellow, and prolate in the blues, but clear 

 throughout. 



<l 



nm 



^=W 



77 



Light enters from an arc lamp, A, or Nernst burner, or the sun, at the slit 

 5, and is collimated by the lens L. Then the parallel rays pass the grating G 

 with its ruled side toward L. From the grating the reflected beam returns 

 to the opaque mirror N, and is then reflected into the auxiliary or adjustment 

 telescope, T. The component beam transmitted at G is reflected from the 

 opaque mirror M, returned to the ruled side of G, and thus also reflected 

 into T coincidently with the other beam. 



107 



