50 W. Gibbs—Miscellaneous Optical Notices. 
1655°6 was found to be — 0°38, and when the scale numbers 4 
ne “gas as the sines or tangents of corresponding angles, 
rithe following data are taken from the more refrangible part 
of the spectrum, the measurements being also those of Ditsch- 
einer, and made with the sam: 
2922°8 489.84 50°84 57" “LL: SOther . 22s. = 
G 2854°7 430°88 50° 37'52"” 43068 1°64334 430°83 
2069-7. -429°00 «. 50°-98' 47" 2-2 1°64352 
In this case the error in the Lg an ae : the middle line, 
2854°7, is —0°20, as determined from the angular deviations, 
and --0:05, as determined from the indices. It must be borne 
in mind that, in this part of the spectrum, the determination 
both of wave lengths and of indices of refraction is difficult on 
account of the feeble intensity of the li 
Since only the differences between the angular deviations of 
the spectral lines are employed in the formulas above given, it 
follows that in determining wave lengths by the method in 
uestion, it is "sat necessary to employ a spectrometer with a 
divited circle and appliances for the measurement of large 
angles. A common spectroscope will be sufficient, if the ob- 
srving telescope be provided with a filar micrometer by means 
of which the angular distances of any given line from two 
uae ae of which the wave — are known may be meas- 
e researches of Angstrém leave nothing to be de- 
ioed as regards the wave-lengths of standard lines, and the 
method given may prove a convenient means of determining 
with all requisite precision the wave lengths of metallic lines. 
§ 2. 
On liquids of high dispersive power.—Of the liquids which have 
hitherto been proposed for the construction of prisms, bisulphid 
of carbon unquestionably Beige the fascia advantages. It 
is cheap, colorless, and unites a moderately high mean refractive 
to a very high iter wer. By tacit consent a prism of 
60° filled witli this liquid has come to be adopted as a sort of 
eee The disadvantages of the bisulphid are equally well 
known, and I have spent no little time and labor in the en- 
deavor es “ind a liquid with a still higher dispersive power, less 
volatile, less sensitive optically to changes ene and | 
less offensive in odor. In these efforts I have not Asien 8 alto- >| 
Bae Se saa oy 
