W. Gibbs—Miscellaneous Optical Notices. 49 
Alison’ in those of low dispersive power, partly because the angu- 
lar deviations to be measured are larger, and partly because the 
spectral lines are less crowded together 
The following example will serve to illustrate the advantage 
of ee shorter intervals :— 
ne Indices. a Indices. A 
cen 518°63 OSE R = | x cae a 1°61882 egaiscns 
26 ral gis3 § 1°62472 517.61 1°61895 51a 3G 
278 517°07 P6Q2479 6s ec 1°61901 apenas 
The data are sah also taken from Van der Willigen’s meas- 
ures with the same prisms. 
When the ales distances between three spectral lines are 
not too great, the angular deviation of the lines may, as I find, 
be substituted for the indices of refraction in eee (1), @) 
and (3). The differences between the an viations are of 
course to be converted into seconds. The ealowing results will 
show the degree of accuracy attainable by this sre. the data 
being taken from Ditscheiner’s* measurements of the indices of 
a flint prism eae Steinheil of Pit cae 60° 4’ 59”. 
Kirchhoff’s scale, Angular deviations. a Indices. 
B 593 687-06 Ue 0 Yee eer SISSS © sesus 
C 694 655°95 47°51’ 19" 655:97 sper 655.82 
ees te ei 2 a oo 
From this it appears that the error in the ed of 
the wave-length of the middle line C is only +0°02 when the 
angular deviations are employed, but amounts to —0°13 when 
the indices of refraction are taken as the elements of we caleu- 
lation. Yet the interval between B and 877 is very 
' The following data are taken from another part of ke ‘scale, 
the measurements being made with the same prism :— 
Kirchhoff’s scale. 2 Angular de viations. a Indices. A 
OS 16483 BIT18 = 40°F 16" es sR sk) eee ete 
1655°6 516°58 49° 4'44” 516°56 Sete 516°61 
16038: 514-08 40° 6.47" = -cc.c. 817 
Hence the error in the determination of be: wave length of 
655°6 is, the | nd angular eee are taken, be len —0°02 
=== 
i » represent 
tions, it seemed worth wis to determine how far for a short 
interval, these could be tgp be Taking the three scale num- 
e error in the wave-length of 
* ee gs der Wellenlingan der Fraunhoferschen Linien des Sonnenspec- 
Am. Jour. Scr.—Seconp pee Vou. L, No. 148.—Jury, 1870, 
