REVERSED AND NON-REVERSED SPECTRA. 97 



14 cm., but this gives ample room for the manipulation of the trough t placed 

 normally in the beam MH. White light enters by way of the collimator SC 

 at any convenient angle 6 (as this does not enter into the equations), and 

 = 6o was used. The opaque mirrors M (and preferably also N) are on 

 micrometers with screws normal to their faces, and each must be provided 

 with adjusting-screws relatively to horizontal and vertical axes. An elastic fine 

 adjustment is desirable. The block contains a number of screw-sockets, b, 

 for attaching subsidiary apparatus. The trough t should preferably be at- 

 tached to an independent supporting arm, not connected with B, and be 

 revolvable about two axes normal to each other. In such a case the position 

 normal to the beam of light may be found from the reverse of motion of the 

 interference rings, while the trough is slowly rotated in a given sense. 



The telescope T (relatively much enlarged in the diagram) is not attached 

 to the block. It is to be used both as a simple telescope for the adjustment 

 of the white slit images to horizontal and vertical coincidence, and as a direct- 

 vision spectroscope. The most convenient attachment for this purpose is 

 the direct-vision prism grating G (film grating) just in front of the objective 

 of T. Two perforated thin disks of brass are useful for this purpose, one disk 

 being firmly attached (like the cap) to the objective, the other to the flat 

 face of the grating with the prism outward. A swivel bolt a, between the 

 disks, thus allows the observer to throw out the grating and use the telescope. 

 A stop arrests the motion of the grating when it is rotated about a, back 

 again, for viewing the spectrum. This plan worked very well, and the el- 

 lipses obtained were magnificent. It was almost possible to control the microm- 

 eter M manually, and all hurtful quiver is absent. The fiducial line to which 

 centers of ellipses, etc., are to be returned is always the sodium doublet pres- 

 ent in sunlight and the arc and artificially supplied by an interposed burner 

 in case of the Nernst lamp. The telescopic lens need not be more than 2 cm. 

 wide, and cross-hairs are not needed. For measuring dispersion the Fraun- 

 hofer lines B, C, D, E, b, F were used. 



5 1 . Equations. The useful equations for present purposes are given in a 

 preceding report,* and the following cases only need be repeated here. If e 

 is the thickness of glass plate of index of refraction /x for the wave-length X, 

 and if the equation fj. = A-{-B/\ 2 , where A and B are constants, be taken as 

 sufficient, 



(i) /i-i 



where A7V is the displacement of the micrometer at the opaque mirror M or 

 N due to the insertion of the plate normally to the component beam in ques- 

 tion. To determine n, B must be known at least approximately. It may be 

 measured in the same adjustment, however, if two Fraunhofer lines are used 

 fiducially. Let dN be the displacement of micrometer to pass the center of 



* Carnegie Inst. Wash. Pub. No. 229, 1915, 40, 41, 42. 



