CHAPTER I. 



METHODS FOR REVERSED AND NON-REVERSED SPECTRUM 



INTERFEROMETRY. 



1. Introductory. Thus far it has been impossible to use the fringes of 

 reversed spectra individually, because of the tremor of the apparatus. It is 

 therefore desirable to endeavor to obviate this annoyance as far as possible, 

 and the end would appear to be most easily obtainable if the distances cor- 

 responding to the same path-difference are made smaller. At the same time 

 the results for small distances will be interesting for this very reason in 

 contrast to the long-distance methods. 



Furthermore, the development of different methods, with a consideration 

 of the peculiarities of each, will constitute an essential contribution to the 

 theory of the phenomena; for from this the degree of importance which is 

 to be attached to the original diffraction at the slit of the collimator (i.e., 

 the limiting angle at the slit, within which diffracted rays must lie to be 

 subsequently capable of interference, whether reversed or inverted) will 

 appear in its relations to the total dispersion of the system. The slit, however 

 fine, is still a wave-front of finite breadth. 



2. Apparatus. In the first experiment, the device with two identical 

 reflecting gratings, GG', figure i , was firmly mounted on a massive spectrom- 

 eter, the four mirrors, m, n, M, N, being specially attached. White light 

 received from the collimator L after two dispersions was viewed at the tele- 

 scope T. Both gratings were on a slide ss, enlarged in figure 2, set in the 

 direction LT of the previous figure. The carriages, figure 2, was provided 

 with universal joints (a with a vertical axis, b and e with horizontal axes 

 normal to each other), while the swiveling of the grating G was controlled 

 by set-screws at d, relative to the axle at e. 



Unfortunately, the displacement of the mirror M, figure i (on a microm- 

 eter) , passes the corresponding pencil across the face of the grating G' and thus 

 virtually includes a fore-and-aft motion of the latter. Thus the fringes pass, 

 with rotation, from very fine, hair-like striations, through a horizontal maxi- 

 mum of coarseness, back to vertical lines again, when homogeneous light 

 and a wide slit are employed. The annoyances due to tremor, however, were 

 not overcome. Moreover, there is difficulty in obtaining Fraunhofer lines 

 normal to the longitudinal axis of the spectrum. This method was therefore 

 abandoned. 



The design shown in figure 3, with a transmitting grating at G (grating 

 space D = 3 5 2 X icr 6 cm.) and a stronger reflecting grating at G' (D = 200 X io~* 

 cm.), was next tested, M being the micrometer mirror. The mean distance of 

 M from N was about 15 cm., from MN to G' about 10 cm. and to G 40 cm. 



9 



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