CHAPTER V. 



ynf 



INTERFEROMETERS FOR PARALLEL AND FOR CROSSED RAYS. 



34. Introduction. Methods. To exchange the component beams of the 

 interferometer, to mutually replace the two pencils which interfere, is not an 

 unusual desideratum, for instance, in the famous experiment of Michelson 

 and Morley. To replace two pencils of component rays, traveling more or 

 less parallel to each other, by pencils moving more or less normal to each other, 

 or to be able to operate upon pencils of corresponding rays (from the same 

 source, crossing each other at any angle) at their point of intersection, may 

 be of interest in a variety of operations to which the interferometer lends 

 itself, or may even suggest novel experiments. The facility with which this 

 may be done, or at least partially done, with the above types of spectrum 

 interferometers, particularly when homogeneous light is used, has tempted 

 me to investigate a number of cases. 



Let us begin with the above diagram- 

 matic method, using two transmitting 

 gratings, G and G', figure 56, with the same 

 (or in general with different) grating con- 

 stants. Let L be the incident beam of 

 collimated homogeneous light, m, n, m', n' , 

 four opaque mirrors on vertical and hori- 

 zontal axes parallel to their faces. The 

 ruled faces of the gratings are to be toward 

 each other. Then the beams Gm and Gn 

 may be reflected either across each other, 

 as shown at win' and nm', thence along n'G' 

 and m'G', and, after a second diffraction at 

 G', unite to enter the telescope at T; or 

 they may be reflected along m, m', and 

 n, n f , parallel to each other, and thereafter take the same course. In the 

 first case homogeneous light is apparently not necessary. It will be seen 

 that the path of the rays is the same, except for the branches mn' and nm' ', 

 and mm' and nn', respectively normal and parallel to each other; moreover, 

 that the rays are exchanged, a and 6 left and right combining at G' in one case, 

 b and a left and right in the other. The rays cross at c in free space and are 

 available there for experiments. Direct light is to be screened off. The ques- 

 tion is whether the mirrors m and n, m' and n', can be adjusted mechanically 

 to move symmetrically toward each other on a vertical axis with sufficient 

 precision to guarantee replacement. This is a matter of trial, though a 

 successful issue is, of course, problematical. It would be advantageous to 

 arrange the experiment so that only one pair of mirrors e.g., m and n need 

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