W. Gibbs on a normal Map of the Solar Spectrum. 7 
sess the beautiful photographic charts of Mr. Rutherfurd extend- 
ing from b to H’. ese charts are not however at present avail- 
able for the determination of wave lengths, because the sae 
upon them beyond G have not yet been identified with li 
whose wave lengths have been measure 
The very numerous measurements of Mr. Huggins* ns the 
relative positions of a great number of spectral lines character- 
istic of particular elements. ‘To give these measurements a real 
value the corresponding wave lengths must be determined asin 
the case of Kirchhoff’s lines. Iam not at present able tostate 
to what extent this has been done, or whether it will be possible 
without a great number of new measurements of wave lengths 
to reduce the scale-numbers given by Mr. Huggins to the wave 
standard. If this can be done, we shall be able to fill up the 
portion of the spectrum beyond the line G not examined by 
irchhoff, and for which Ditscheiner’s measurements are not 
available. + 
In any event the tables and formula above given will enable 
an observer to determine the wave length of any new spectral 
line by simple comparison with Kirchhoft’s chart, provided only 
that the line lies between B and G. In practice I suggest that 
this comparison may be readily made directly. Let for example 
three or four flint glass prisms be used with the spectroscope 
as to give an amount of es pp apse vases equal = that 
of Kirchhoff’s apparatus. If no ale telescope be employed 
with an object glass of about 18 ao focal length, it Pi be 
easy to place Kirchhoff’s chart in such a position that the re- 
flected image of any small portion of it shall be in the field of 
view of the meni et 5 telescope at the same time with the direct 
eo ee 
Weis 
aoe 
* Philosophical ——— for 1864; also Pogg. Ann., exxiv, p. 2 ie 
ce the above was written I have succeeded, by comparing ae lines upon 
i anaelen scale wit ‘hanes upon Kirchhoff’s scale, in constructing interpolation 
together with Prin lengths of all the elements given 
—W.G. Nov. lst : he 
