RECENT PROGRESS IN OPTICS. 119 



the field of view. According to the theory of interference the difference 

 of path between the distances from one face of the plate to the two 

 mirrors should be small; beyond a certain limit interference phenomena 

 vanish, and this limit is smaller in proportion as the light is more com- 

 plex. In the case of approximately homogeneous light there are peri- 

 odic variations of distinctness in the fringes. For example, assume 

 sodium light, which in the spectroscope is manifested as a pair of yellow 

 lines near together. In the refractometer there are two sets of inter- 

 ference fringes, one due to each of the two slightly different wave 

 lengths. When the difference of path is very small, or nearly the same 

 for both of these radiation systems, the fringes coincide. The wave 

 length for oue is about one thousandth less than that for the other. If 

 the difference of path is about five hundred waves, the maximum of 

 brightness for one system falls on a minimum of brightness for the 

 other, and the fringes become faint. They become again bright when 

 the difference of path reaches a thousand wave lengths. The case is 

 entirely similar to the familiar production of beats by a pair of slightly 

 mistimed forks. 



The method of interference thus furnishes through optical beats a 

 means of detecting radiation differences too minute for resolution by 

 ordinary spectroscopic methods. Spectrum lines are found to be double 

 or multiple when all other means of resolving them fail; and the diffi- 

 culty of attaining truly homogeneous light is far greater than was a 

 few years ago supposed. By the new method it becomes possible to 

 map out the relative intensities of the components of a multiple line, 

 their distance apart, and even the variations of intensity within what 

 has for convenience been called a single component. Each of the two 

 sodium lines is itself a double whose components are separated by an 

 interval about one-hundredth of that between the long-known main 

 components; and an interval yet less than one- fifth of this has been 

 detected between some of the components of the green line of mercury. 

 Indeed Michelson deems it quite possible to detect a variation of wave 

 length corresponding to as little as one ten-thousandth of the interval 

 between the two main sodium lines (Astronomy and Astrophysics, p. 100, 

 February 1894). 



This new-found complexity of radiation, previously thought to be 

 approximately if not quite simple, proved to be a temporary barrier to 

 the accomplishment of the plan of using a light wave as a standard of 

 length. It necessitated careful study of all those chemical elements 

 which give bright lines that had been supposed to be simple. The red 

 line of cadmium has been found the simplest of all those yet examined. 

 The vapor in a rarefied state is held in a vacuum tube through which 

 the electric spark is passed, and under this condition the difference of 

 path for the interfering beams in the refractometer may be a number 

 of centimeters. A short intermediate standard, furnished with a mirror 

 at each end, is now introduced into the comparer, and moved by means 



