W. LeConte Stevens—Recent Progress in Optics. 279 
surpassed? If so, it must be not so much by increased care in 
measurement as by increase of delicacy in the means employed. 
The principle applied in the use of the interferential comparer 
is simple enough; the mode of application cannot be clearly 
indicated without a diagram, but probably all physicists have 
seen this diagram, for it was first brought out eight years ago.* 
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 mirrors is movable in 
front of a micrometer screw, whose motion causes these 
fringes to move across the telescopic field. If the light be 
absolutely homogeneous, the determination consists in the 
measurement of the distance through which the movable mir- 
ror is pushed parallel to itself and the counting of the number 
of fringes which pass a given point in 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 complex. In the case of approximately homog- 
eneous light there are periodic variations of distinctness in 
the fringes. For exainple, 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 leneths. 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 one 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 sys- 
tem 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 mistuned 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 difficulty of attaining truly homo- 
geneous light is far greater than was a few years ago supposed. 
_ By the new method it becomes possible to map out the rela- 
tive intensities of the components of a multiple line, their dis- 
tance apart, and even the variations of intensity within what 
* Ibid., December, 1887, p. 427. 
