REGISTRATION OF THE CHEMICAL ACTION OF TOTAL DAYLIGHT. 619 
by the two methods agree. The black line represents the intensity as determined by 
the pendulum instrument, the dotted line that obtained by the new photometer, the 
abscissae giving the times of observation, and the ordinates the chemical intensity in 
the terms of the unit above described. The mean chemical intensities, as observed on 
the above days by the two methods, are represented by the following numbers, for the 
definition of which the reader is referred to page 621. 
Daily Mean Chemical Intensity. 
Plate XXVIII. 1. Pendulum photometer. 2. New instrument. 
Fig. 7, April 29, 1864 . . 
. 62-0 
62-3 
Fig. 8, May 10, 1864 . . 
. 41-3 
43-3 
Fig. 9, June 8, 1864 . . 
, . 64-7 
65-3 
From these results the agreement of the two methods is well seen. 
As a second test of the trustworthiness and availability of the method for actual 
measurement, I give the following results of determinations, made at the same time and 
on the same spot, by two observers with two of the new instruments. These determi- 
nations, made with the two graduated fixed strips B and C (page 613), were conducted 
in every way independently, so that the results serve as a fair sample of the accuracy 
with which the measurements can be practically carried out. 
Simultaneous Determinations made independently with two Instruments by two observers. 
July 11, 1864. 
July 15, 1864. 
Time. 
Chemical Intensity. 
Time. 
Chemical Intensity. 
Instrument 1. 
Strip B. 
Instrument 2. 
Strip C. 
Instrument 1. 
Strip B. 
Instrument 2. 
Strip C. 
h 
m 
h 
m 
10 
30 A.M. 
0-16 
0-14 
10 
0 A.M. 
0-16 
0-17 
0-14 
0-14 
10 
1 
0-19 
0-19 
io’ 
31 
0-14 
0-15 
11 
0 
0-049 
0-046 
0-12 
0-13 
11 
1 
0-049 
0-046 
io’ 
32 
0-13 
0-11 
11 
35 
0-12 
0-12 
y 
9 
0-15 
0-12 
99 
0-12 
0-12 
10 
33 
0-14 
0-12 
11 
36 
0-12 
0-13 
li 
0 
0-13 
0-12 
0-11 
0-11 
12 
0 
0-31 
0*27 
12* 
30 p.m. 
0-13 
0-10 
12 
30 p.m. 
0-31 
0-29 
0-13 
0-12 
12 
31 
0-38 
0-37 
99 
0-14 
0-13 
12 
32 
0*33 
0-31 
0-14 
0-12 
12 
33 
0-35 
0-32 
\ 
’ 0 
0-17 
0-17 
1 
5 
0-13 
0-13 
0-18 
0-18 
2 
0 
0-27 
0-25 
2 
*30 
0-057 
0-060 
0-27 
0-25 
0-068 
0-070 
3’ 
’lO 
0-24 
0-23 
3’ 
30 
0-059 
0-057 
3 
11 
0-21 
0-24 
0-067 
0-062 
3 
12 
0-18 
0-23 
3 
*31 
0-063 
0-045 
3 
13 
0-17 
0-18 
0-054 
0-045 
3 
40 
0-24 
0*23 
A 
20 
0*028 
0-025 
3 
41 
0-14 
0-15 
0-028 
0-025 
4 
0 
0-21 
0-20 
0-032 
0-028 
4 
30 
0-11 
0-13 
0-14 
0-14 
A 
31 
0-14 
0-15 
0-15 
0-14 
4’ 
32 
0*16 
0-14 
