492 Mr. J. Barnes on the Analysis of 
opening, and hence a number of exposures made upon one 
plate. 
With the silvered plates illuminated in this way, with 
divergent light, the entire rings of the interference Ramee 
are observed in the focal plane of the objective. as shown on 
Plate XXV. fig. 1. The following method, however, was 
found to be better for the analysis of he radiation The lens 
L was removed and the interferometer placed directly behind 
the prisms so that the parallel light fell upon the silvered 
plates. ‘With a broad slit in the spectroscope we have in 
the telescope, focussed for infinity, broad lines corresponding 
to the lines in the spectrum. ‘These lines are crossed with 
the interference-bands produced by the plates. By this 
means the light has been concentrated into a few interference- 
bands and on this account many of the weaker components 
appear which cannot be seen with the light divergent as above. 
Fig. 2 shows this clearly. This photograph is of the bright 
green mercury radiation, and shows three components wiea 
the interference-plates are separated 8 mms. 
This method possesses also another great advantage. Due 
to the number of lines in most spectra we have in the field of 
the telescope at the same time a number of lines containing 
different kinds of interference-bands depending upon the 
constitution of the radiation making up each Bice. This 
facilitates greatly the analysis of the radiations and we see at 
once any change that may take place in one or all of the lines 
through any change of external conditions. The dispersion 
of the prisms and ihe magnification of the telescope were such 
that about half of the spectrum was visible at once. Plate 
XXYV. figs. 3, 4, and 5 each show the interference-bands due to 
the two yellow anil the green lines of mercury vapour taken 
at the same time with the plates separated different distances. 
On account of the broad slit the yellow lines passed through 
the interference-plates together, and hence their interference- 
bands are superimposed upon one another. ‘The other lines 
in this region of the spectrum of mercury being of less 
intensity do not show in the photographs, which were exposed 
only long enough to get the clearest pictures of the lines 
considered. The dark green line was quite visible to the eye 
after passing through the silvered plates. The curvature of 
the bands in the different lines is of course due to the amount 
of separation of the plates, and to the angle of incidence with 
which the radiations are incident upon the interferometer- 
plates. 
For the determination of the scale-reading corresponding 
Loo) 
to the place where the silvered plates were in contact, a 
