Egon S. Pearson 
87 
The variations in judgment in the Estimating Experiment are very large com- 
pared with those in the other, and at the same time there is low correlation 
between successive judgments, so that the observations will be found to be scattered 
far more nearly in accordance with the Normal Error Law than in the three 
preceding experiments. In the case of the Counting Experiment, the skew distri- 
bution of the 1000 observations has already been referred to. But for one or two 
exceptions (as III and XIX) the individual series in the Counting conform more 
closely to a general type than in the Trisections or Bisections, and this results in 
the very smooth values of the constants and R^'. 
X. Experiment E. Plate measurements with Zeiss Comparator. 
The values of p-^ only have been calculated ; these, with and a brief description 
of the nature of the marking measured, are given in the Table XVII ; o-j is in 
millimetres. Series I — VIII involved settings of both slide and micrometer, IX of 
micrometer only. 
No great weight can be attached to the result of one series of .50 readings on a 
marking, but it is justifiable to draw certain conclusions from the results of the 
eight series. In the first place, there appears to be a significant correlation between 
the successive measures of the edge of a band (I and II), but in measuring the 
centres, i.e. in bisecting a bright maximum with the cross wire, there is on the 
whole no correlation. This perhaps might be expected ; the edges of bands or 
maxima in photographic spectra are not quite sharply cut, so that some uncertainty 
must exist in the observer's mind as to where the real edge should be taken to be ; 
his opinion on this point may vary throughout the course of the sitting, and con- 
sequently correlation will be found between the successive readings. On the other 
TABLE XVIL 
Series 
pi 
<^i 
Description of marking 
I 
II 
III 
IV 
V 
VI 
VII 
VIII 
IX 
4- -.384 ± -081 
+ -467 ± -075 
+ -117+ -094 
+ •090+ -095 
+ -021 + -095 
- -001 ± -095 
- -050 + -095 
+ -227 ± -091 
+ -288 + -087 
•0016 
■0015 
•0007 
•0012 
•0016 
•0019 
•0022 
•0041 
•0004 
Sharp edge of bright band ) „ , 
Slightly \-aguer edge than I \ ^"^S^ 
Clear and nai'rovv maximmn^ 
n 11 11 
Broad and ol:)seure „ 
„ „ soft „ 
11 11 11 11 1 
Microninter screw settings only 
hand, in the bisection of a narrow maximum, there will be little doubt as to the 
position of the centre ; the real estimate of the observer will vary but slightly, and 
the variations in the reading will be due mainly to failure in breaking off the push 
