On a Mica Echelon G rati ng. G27 



applies when using the prism in connexion with the spectro- 

 meter. 



To see the dispersion curve by means of the grating and 

 prism, one has only to view a naked arc-lamp through the 

 small rectangular aperture. 

 University of Wisconsin. 



LXII. A Mica Echelon Grating. By Prof. P. W. Wood*. 



HAYING experienced some difficulty, when discussing 

 Michelson's remarkable retardation-grating, in making- 

 students understand how it is possible for the sodium lines to 

 be separated by a distance fifteen or twenty times as great as 

 the distance between the spectra, it struck me that an echelon, 

 built up of yery thin films instead of thick plates, coming 

 midway between the ordinary grating and the echelon as 

 commonly constructed, would be useful in demonstrating the 

 theory. 



Such a grating I have made of mica. 



By it lines., which with an ordinary grating of the same 

 number of grooves would appear single, can be resolyed and 

 still not be farther apart than the spectra. It shows spectra 

 of the same general appearance as in the more powerful 

 instruments, can be set for single and double order, and 

 though useless as a tool for research, is almost as satisfactory 

 for purposes of demonstration as the costly batteries of thick 

 plates. 



A number of thin sheets of mica were examined with the 

 interferometer, and one selected oyer a considerable portion 

 of which the fringes appeared straight and unbroken. This 

 area was roughly outlined w 7 ith a pin-scratch, and cut up 

 into a dozen small rectangles with a print-trimmer. The 

 retardation of one of these was measured with the interfero- 

 meter, and found to be fifty wave-lengths for sodium-light. 

 The grating would therefore yield spectra of about the 50th 

 order : I say " about," for the order varies with the wave- 

 length of the light and the inclination of the grating. A 



* Communicated by the Physical Society : read February 8, 1901. 



