, 
178. Ee Langley— Determination of Wave-lengths 
the large original charts* which have been drawn by Mr. J. E. 
Keeler, of this observatory, and which seem to me favorable 
" specimens of the accuracy attainable by this method. 
e are now prepared to test the accuracy of the varius 
formule connecting refraction with wave-length, though it wi 
be convenient to first prepare a table showing what this rela- 
tion is in the visible part of the spectrum of the prism employed. 
In the following table the deviations in the visible spectrum 
were measured by the spectrometer, reading to 10” of are, and 
which has been already described, in which for this special 
purpose, the bolometer was replaced by an achromatic observ- 
ing telescope with a micrometer eye-piece, and the indices of 
refraction were computed by the usual formula. ‘“O” in the 
ultra-violet was measured by aid of a Soret fluorescent eye- 
piece, and its wave-length is from Cornu. The other wave- 
lengths are taken from Angstrém, but the unit is here, the 
micron = ;;/;5 millimeter = (10,000 times the unit of Ang- 
strém’s scale). “A” is here the symbol for the wave-length. 
The following indices in the visible spectrum, on which the 
computations for testing the formule are founded, are trust- 
worthy to the fourth decimal place here given: 
Tasce III.— Observed indices in visible spectrum of Hilger prisi- 
Line. d n 
A 076009 46° 49” 05" 15714 
Occ ue cere 0°65618 47° 15’ 45" 15757 
Dire sage .... 0°58890, 47° 41/ 15” 15798 
Wide os 0°51667 48° 21’ 05 175862 
es 0°48606 48° 44’ 15" 15899 
1 ep eaten pene lh 0°39679 50° 34’ 05 1°6070 
og cate Ge 0°34400 52° 43’ 00” 1°6266 
_A smooth curve, drawn through points whose positions are 
given by the above table, represents with accuracy the relation 
between n and 2 in the visible part of the spectrum. This 
method is however obviously inapplicable to the very extended | 
invisible portion, below the A line, and accordingly attempt 
e t made to effect the determination o corresponding 
indices and wave-lengths, by extending the curve derived from 
the above observations by means of formuls. Several formule 
have} it will be remembered, been proposed by physicist ex- 
w 
pressing » as a function of A, and containing constants | ich 
are to be determined by observation. But it has never hitherto 
* These original charts were exhibitel to the members of the National Academy 
of Sciences, at Washington, in April, 1883. en here given in illustra 
possible by the originals. 
tion 
e engravi , germ 
scale, will merely indicate the exactuess of interpolation oe 
ah 
ae ae 
