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. 321
This expression vanishes for = +X/s which is also the value of dco, the distance
between the spectra.
Hence in general there are two spectra visible as indicated in Fig. 4.
277 -7 O;7 207
FIG. 4.
By slightly inclining the echelon one of the spectra is readily brought to the centre
of the field, while the adjacent ones are at the minima, and disappear. The remaining
spectra are practically invisible, except for very bright lines.
As has just been indicated, the proximity of the successive spectra of one and the
same line is a serious objection, and as this proximity depends on the thickness of the plates
-which for mechanical reasons cannot well be reduced below 5 or 6 mm.-it is desirable
to look to other means for obviating the difficulty, among which may be mentioned the use
of a liquid instead of air.
In this case formula (II) becomes
de t 1 C (ICL - ( l)1 t
dcx/X s - ( - -, - X) cX j= cSand formula (IV) becomes
dO x
dm /-1S
Repeating the same operations as in the former case, we find
nct'
and E = -.
ct
The limit of resolution is still the nth part of the distance between the spectra,
but both are increased in the ratio b/c.
Suppose for instance the liquid is water. Neglecting dispersion the factor would
be 3 55. Hence the distance between the spectra will be increased in this proportion,
but the limit of resolution will also be rnultiplied by this factor. But as there is now
a surface water-glass which reflects the light, the loss due to this reflection will be
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