PHYSICS: C. BARUS 
121 
which may be called the corresponding air displacement 
bN - bNa 
Jj = ^ — 
Here dN = 8Na = N - - (Na - = N - Na - {N' - N\) 
so that the difference of the corresponding positions of micrometer for 
a given Fraunhofer line, with and without the plate, are to be found. 
The method is quite accurate as will be seen below. Mpre than two 
constants A and B may be taken if desirable, with 3 Fraunhofer lines. 
To return to equation (1), remembering that 2jS/X2is small, it is seen 
that the percentage accuracy oi n — 1 and A N are nearly equal. Now 
A iV for a plate 5 to 6 mm. thick and ordinary glass is about 0.3 cm. 
This may be measured within 10-^ cm. or 3 parts in 10^ of A iV or one 
or two units in the fourth place of /x the index of refraction of the glass. 
A much more serious consideration is the consistent measurement of 
the thickness of plate e, which must be given to less than 10~^ cm. if 
the same accuracy is wanted. Naturally this presupposes optic plate. 
Hence the data will be inaccurate as to absolute values from this cause. 
The plates used frequently showed increases of thickness of several 
10~3 cm. within a decimeter of length. Absolute values were however 
without interest in this paper. 
To show that less than 10~^ cm. is guaranteed on the micrometer 
in the placing of elliptic centers at the sodium line, the following pairs 
of results, made at different times and with entirely independent adjust- 
ments may be cited. The screw pitch was 0.025 cm. and the drum 
divided into 50 parts with a vernier to 0.1 part. 
FRAUNHOFER LINE 
PITCH 
DRUM 
PITCH 
DRUM 
DIFFERENCE 
B 
X85 
17.1 
74 
33.2 
cm. 
0.25000+0.01695 
C 
X85 
23.3 
74 
39.3 
0.25000+0.01700 
D 
X85 
41.0 
75 
7.0 
0.25000+0.01700 
E 
XX86 
14.9 
75 
30.9 
0.25000+0.01700 
b 
XX86 
19.2 
75 
35.2 
0.25000+0.01703 
F 
XXX86 
36.6 
76 
2.6 
0.25000+0.01703 
X ellipses long horizontally X X circles XXX ellipses long vertically. 
The total difference is less than 10~^ cm. and probably due to the width 
of the Fraunhofer lines with deficiency of hght at the ends of the spec- 
trum. Apart from the measurement of the thickness e therefore, the 
method is guaranteed for fourth place work. 
Hence 
(4) 
