( 248 ) 
cession of tests of gradually increasing difficulty would be of 
especial value. 
I remembered these remarks as I tested the very original echelon, 
invented by MICHELSON. 
The echelon at my disposition, made by Hrreemr, London, consists 
of thirty plates each about 7,8 m.m. thick, made of light flint-glas, 
set with 1 m.m. steps. A clear aperture of 1 m.m. is left beyond 
the width of the largest glassplate. The number of apertures », 
operative in the formation of the spectrum is hereby one more than 
the number of plates. The mounting was somewhat improvised. 
Telescope and collimator belonging to a KiRCHHOFF spectroscope 
were employed. The telescopes had object-glasses of 50 cm. focus 
and 38 m.m. aperture. It is evident that in order to get greater 
intensity, glasses of shorter focus would have been preferable. 
Denoting by dd, the difference of wave-length of spectral lines 
when they are just distinguishable as separate in the spectroscope, 
by t the thickness of the plates of glass, and by » the above mentioned 
number then, we know 
RETE pare we 
ET TRR te Beer |G 
if 
du 
=(u —1)— 
be nn 
The resolving power is given by 
wal À aoe knt 5 
Dn 6) 
For the green line 4 =5460 A. M. we obtain in the case of 
CRs alates: dd, 
nT ) 2 ZZ IQ ‘ 3 4 ae 
our echelon 7 5460. 10=7 80000 and 44 7 ‚6. 10 
In the calculation of # I used the following values of the refractive 
indices given to me by HILGER 
uo = 1.5713 
Mp = bos 
(see 
Me == 1.5936 
