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.

PROF. MICHELSON, THE ECHELON SPECTROSCOPE. 317
the "visibility curve" is complete, and analyzed either by calculation or by an equivalent
mechanical operation.
Notwithstanding these difficulties, it was possible to obtain a number of rather interesting results, such as the doubling or the tripling of the central line of Zeeman's triplet, and
the resolution of the lateral lines into multiple lines; also the resolution of the majority
of the spectral lines examined, into more or less complex groups; the observation of the
effects of temperature and pressure on the width of the lines, etc.
It is none the less evident that the inconveniences of this process are so serious
that a return to the spectroscopic methods would be desirable if it were possible
(1) to increase the resolving power of our gratings; (2) to concentrate all the light in
one spectrum.
It is well known that the resolving power of a grating is measured by the product
nzn of the number of lines by the order of the spectrum. Attention has hitherto been
confined almost exclusively to the first of these factors, and in the large six-inch grating
of Prof. Rowland there are about one hundred thousand lines. It is possible that the
limit in this direction has already been reached; for it appears that gratings ruled on
the same engine, with but half as many lines, have almost the same resolving power
as the larger ones. This must be due to the errors in spacing of the lines; and if
this error could be overcome the resolving power could be augmented indefinitely.
In the hope of accomplishing something in this direction, together with Mr S. W.
Stratton, I constructed a ruling engine in which I make use of the principle of the
interferometer in order to correct the screw by means of light-waves from a homogeneous
source. This instrument (only a small model of a larger one now under construction)
has already furnished rather good gratings of two inches ruled surface, and it seems
not unreasonable to hope for a twelve-inch grating with almost theoretically accurate
rulings.
As regards the second factor-the order of the spectrum observed, but little use
is made of orders higher than the fourth, chiefly on account of the faintness of the
light. It is true that occasionally a grating is ruled which gives exceptionally bright
spectra of the second or third order, and such gratings are as valuable as they are
rare, for it appears that this quality of throwing an excess of light in a particular
spectrum is due to the character of the ruling diamond which cannot be determined
except by the unsatisfactory process of trial and error.
If it were desirable to proceed otherwise-to attempt to produce rulings which
/ /,/ / / /
/ / /! / f
/ /, / / / /
/ / / / / / /
FIG. 1.
should throw the greater part of the incident light in a given spectrum, we should try
to give the rulings the form shown in section in Fig. 1.