Memoirs presented to the Cambridge philosophical society on the occasion of the jubilee of Sir George Gabriel Stokes, bart., Hon. LL. D., Hon. SC. D., Lucasian professor.

XV. The Echelon Spectroscope. By Professor A. A. MICHELSON, Sc.D.
[Received 19 October 1899.]
THE important discovery of Zeeman of the influence of a magnetic field upon the
radiations of an approximately homogeneous source shows more clearly than any other
fact the great advantage of the highest attainable dispersion and resolving power in the
spectroscopes employed in such observations.
If we consider that in the great majority of cases the separation of the component
lines produced by the magnetic field is of the order of a twentieth to a fiftieth of the
distance between the sodium lines, it will be readily admitted that if the structure of the
components themselves is more or less complex, such structure would not be revealed by
the most powerful spectroscopes of the ordinary type.
In the case of the grating spectroscope, besides the difficulty of obtaining sufficient
resolving power, the intensity is so feeble that only the brighter spectral lines can be
observed, and even these must be augmented by using powerful discharges-which usually
have the effect of masking the structure to be investigated.
Sorne years ago I published a paper describing a method of analysis of approximately
homogeneous radiations which depends upon the observation of the clearness of interference
fringes produced by these radiations. A curve was drawn showing the change in clearness
with increase in the difference of path of the two interfering pencils of light,-and it was
shown that there is a fixed relation between such a "visibility curve" and the distribution
of light in the corresponding spectrum-at least in the case of symmetrical lines*.
It is precisely in the examination of such minute variations as are observed in the
Zeeman effect, that the advantages of this method appear,-for the observations are entirely
free from instrumental errors; there is practically no limit to the resolving power; and
there is plenty of light.
There is however the rather serious inconvenience that the examination of a single line
requires a considerable time, often several minutes, and during this time the character of
the radiations themselves may be changing.
Besides this, nothing can be determined regarding the nature of these radiations until
* In the case of asymmetrical lines another relation is necessary, and such is furnished by what may be called
the "phase curve."