118 RECENT PROGRESS IN OPTICS. 



emphasize that this association is made up of Americans; but, with 

 full recognition of the greater spread of devotion to pure science in 

 Europe, of the extreme utilitarian spirit that causes the value of nearly 

 every piece of work in America to be measured in dollars, we are still 

 able to present work that has challenged the admiration of Europe, 

 that has brought European medals to American hands, that has been 

 done with absolute disregard of monetary standards; work has been 

 recognized, even more in Europe than in America, as producing defi- 

 nite and important additions to the sum of human knowledge. 



In drawing attention to some of this work it will be a pleasant duty 

 to recognize also some that has been done beyond the Atlantic — to 

 remember that science is cosmopolitan. The starting point is neces- 

 sarily arbitrary, for an investigation may last many years and yet be 

 incomplete. To note recent progress, it may be important to recall 

 what is no longer recent. 



LIGHT WAVES AS STANDARDS OF LENGTH. 



You are therefore invited to recall the subject of an address to which 

 we listened in this section at the Cleveland meeting in 1888, when 

 Michelson presented his "Plea for light waves." In this he described 

 the interferential comparer, an instrument developed from the refrac- 

 tometer of Jamin and Mascart, and discussed various problems which 

 seemed capable of solution by its use. In conjunction with Morley he 

 had already used it in an inquiry as to the relative motion of the earth 

 and the luminiferons ether (American Journal of Science, May, 1886, 

 p. 377), and these two physicists together worked out an elaborate series 

 of preliminary experiments (ibid., December, 1877, p. 427) with a view 

 to the standardizing of a metric unit of length in terms of the wave 

 length of sodium light. By use of a Eoland diffraction grating, Bell had 

 determined the sodium wave length with an error estimated to be not 

 in excess of one part in two hundred thousand (American Journal of 

 Science, March, 1887, p. 167). Could this degree of accuracy be sur- 

 passed? If so, it must be not so much by increased care in measure- 

 ment as by increase of delicacy in the means employed. The principle 

 applied in the use of the interferential comjmrer is simple enough ; the 

 mode of application can not be cleary indicated without a diagram, but 

 probably all physicists have seen this diagram, for it was first brought 

 out eight years ago (ibid., December, 1887, p. 427). By interference of 

 beams of light, reflected and transmitted by a plate of plane parallel 

 optical glass, and then reflected back by two mirrors appropriately 

 placed, fringes are caught in an observing telescope. One of the mir- 

 rors is movable in front of a micrometer screw, whose motion causes 

 these fringes to move across the telescopic field. If the light be abso- 

 lutely homogeneous, the determination consists in measurement of the 

 distance through which the movable mirror is pushed parallel to itself 

 and the counting of the number of fringes which pass a given point in 



