±02 C. Bar us — Methods in Reversed and 



Art. XLII. — Methods in Reversed and Non-reversed Spec- 

 trum Interferometry ; by Cakj, Barus.'* 



1. Introductory. — Thus far it had been impossible to use the 

 fringes individually, because of the tremor of the apparatus. 

 It is therefore desirable to endeavor to obviate this annoyance, 

 if possible, and the end would appear to be most easily obtain- 

 able if the distances 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 (meters). 



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 wavefront of 

 finite breadth. 



2. Ap2~>aratiis. — In the first experiments, the device with 

 two identical reflecting gratings, GG', fig. 1, was firmly 

 mounted on a massive spectrometer, the four mirrors m, n, 

 M, JY, being specially attached. White light received from 

 the collimator, L, after two dispersions, was viewed at the 

 telescope T. Both gratings were on a slide ss, enlarged in 

 fig. 2, set in the direction LT of the previous figure. The 

 carriage, c, fig. 2, was provided with universal joints (a with a 

 vertical axis, b and e with horizontal axes normal to each other), 

 while the swivelling of the grating G was controlled by set 

 screws at d, relative to tbe axle at e. 



Unfortunately the displacement of the mirror M (on a 

 micrometer) passes the corresponding pencil across the face of 

 the grating G' aud thus virtually includes a fore and aft 

 motion of the latter. Thus the fringes pass, with rotation, 

 from very tine hairlike striations, through a horizontal maximum 

 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 dif- 

 ficulty in obtaining Fraunhofer lines normal to the longitudinal 

 axis of the spectrum. This method was, therefore, abandoned. 



The design shown in fig. 3, with a transmitting grating 

 at G (grating space D = 352 X 10"° cm.) and a stronger reflect- 

 ing grating at G' {D = 200 X 10"' cm.), was next tested, M 



* Abridged from a Report to the Carnegie Institution of Washington, 

 D. C. 





