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.

322 PROF. MICHELSON, THE ECHELON SPECTROSCOPE.
very much less, so that it will be possible to employ a greater number of elements,
thus restoring the resolving power. At the same time the degree of accuracy necessary
in working the plates is 3a55 times less than before.
For many radiations the absorption due to thicknesses of the order of 50 cm. of
glass would be a very serious objection to the employment of the transmission echelon.
I have attempted therefore to carry out the original idea of a reflecting echelon, and
it may be of interest to indicate in a general way how it is hoped the problem may
be solved.
Among the various processes which have suggested themselves for realising a reflecting echelon, the following appear the most promising:
In the first a number of plates, 20 to 30, of equal thickness, are fastened together
as in Fig. 5, and the surfaces A and B are ground and polished plane and parallel.
They are then separated and placed on an inclined plane surface, as indicated in Fig. 6.
A
FIG. 5.
FIG. 5.
FIG. 6.
If there are differences in thickness of the air-films the resulting differences in the
height of the plates will be less in the ratio tana. An error of X/n may be admitted
for each plate-even in the most unfavorable case in which the errors all add; and
consequently the admissible errors in the thickness of the air-films may be of the order