278 W.,LeConte Stevens—Recent Progress in Optics. 
a 
optics has of late become transferred in large measure to the 
domain of the psychologist; but he in turn has adopted many 
of the instruments as well as the methods of the physicist. 
The two cannot afford to part company. If I feel particularly 
friendly to the psychologist, more so than can be accounted 
for by devotion to pure physics, it may be fair to plead the 
influence of old association. If Iam known at all in the scien- 
tific world, the introduction was accomplished through the 
medium of physiological optics. But, with the limitations 
imposed, it is not possible even to do justice to all who have 
done good work in optics. If prominence is assigned to the 
work of Americans, it is not necessary to emphasize that this 
Association is made up of Americans; but, with full recog- 
nition 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 that has been recognized, even 
more in Europe than in America, as producing definite 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 necessarily 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 refractometer 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 
luminiferous ether,* and these two physicists together worked 
out an elaborate series of preliminary experimentst 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 Rowland 
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.t Could this degree of accuracy be 
* This Journal, May, 1886, p. 377. 
+ Ibid., December, 1887, p. 427. 
} Ibid., March, 1887, p. 167. 
