W. Gibbs—Miscellaneous Optical Notices. 47 
wave length, according to Van der Willigen, and the fourth 
the wave length as found by formula (2) by combining B with 
each line after C in succession. 
The mean of the seven values of the wave length of C thus 
found is 656°70, which is in excess of Van der Willigen’s own 
determination of the value of C by 0:14. From this it appears 
in the computed vuhies of ©, as the emevalk between B and 
the second line of comparison are increased, will however clear- 
ly appear from the table. The following results were obtained 
with the indices of a second Steinheil prism, No. 2, of 46° 52’ 
25/78, ie of flint glass. 
1°60521 B and 8a 656°21 —0°35 
6 1°60694 B and 11 656°33 —0°23 
8a 1°60872 B and 13 656°31 —0°25 
it 1°60973 B and 16 656°38 —0°18 
13 1°60998 B and 17 656°47 —0.09 
16 1°61408 
17 161495 
The mean of which is 656-28, the error se ieing —0°28, To deter- 
mine to what extent the method applies, when flint glass prisms 
are used, and the indices are selected from the more refrangible 
portion of the spectrum, the following data were assumed :— 
162917 48639 FandG 43888 -+0.30 
35 1°63244 467°00 Fand39 438-76 +018 
37 1°63828 43858 Fand38 438°82 -+0°24 
38 163981 43428 Gand35 438-76 -+0°18 
39 163965 432°74 35 and 38 438°75 +017 
= — sets 35 and 39 — seta 
formula (2), and the miles obtained oe combining the other 
lines i iven in col 5 and i 
that, as in the case of the less refrangible portion of the spec- 
the results abe are with this prism always too high. 
For the purpose of comparison, I have computed the same wave . 
length from the indices of refractio on of the second prism. The 
data and results are as follows :— 
F 162382 48639 FandG@ 43907 -Lo-49 
35 162657 9 467:00 9 Fand39 «= 43889 4.031 
37 163221 43858 Fand38 48892 0-34 
38 1763824 43428 Gand35 439:00 +0-49 
39 163358 43274 35 and38 43889 10-31 
G 163400 48112 85and39 43884 10-26 
