INTENSITY OF THE CHEMICAL ACTION OF TOTAL DAYLIGHT. 
659 
Standard sensitive papers thus exposed under the several cylinders were then taken 
into the dark room, and the points of identical tint on the fixed strip then read off. In 
this way a number of points are found upon the fixed strip, the relative intensities of 
which are known. The normal tintf (of which the intensity =1) is next read off on 
the strip ; and if its reading corresponds to any one of the tints previously read off, the 
true value of that tint is known to be that of unity, and those of the other tints can 
readily be ascertained. Several experiments with the cylinders are made for each strip 
to be calibrated, and in one or more of these a tint is sure to be found equal to the 
normal tint. As an example I give the following data obtained in the calibration of a 
fixed strip marked C, on which the mean reading of the normal tint 1=1 was found to 
lie at 132 millims. 
Column I. contains the numbers of the cylinders under which the papers were ex- 
posed, column II. the mean reading of these tints on the fixed strip, column III. the 
corresponding relative intensities, and column IV. the true intensities when the tint of 
reading, 132 millims., is taken equal to the normal. 
Experiment A. 
I. 
II. 
III. 
IY. 
2. 
170 
2-32 
0-27 
3. 
163 
4-00 
0-46 
4. 
151 
6T3 
0-70 
5. 
131* 
8-72 
1-00 
6. 
101 
11-75 
1-35 
Experiment B. 
2. 
168 
2 32 
0-29 
3. 
158 
4-00 
0-52 
4. 
146* 
6-13 
0-79 
6. 
82 
11-75 
1-51 
Experiment C. 
1 . 
166 
1-00 
0-31 
2. 
150* 
2-32 
0-72 
3. 
106 
4-00 
1-24 
Experiment D. 
I. 
II. 
III. 
IY. 
1 . 
173 
1-00 
0-28 
2. 
157 
2-32 
0-65 
3. 
119* 
4-00 
1-12 
4. 
59 
6-13 
1-72 
5. 
21 
8-72 
2-44 
Experiment E. 
1 . 
161 
1-00 
0-44 
2. 
130* 
2-32 
1-02 
3. 
70 
4-00 
1-76 
4. 
15 
6-13 
2-69 
Experiment F. 
4. 
147* 
6-13 
0-77 
5. 
123 
8-72 
1-09 
The reading marked with an asterisk in each case is the one used to connect that ex- 
periment with the others. In cases in which no reading occurs identical with the one 
in previous experiments where the true intensities have been found, the true intensity 
of the nearest reading is obtained by interpolation from those of two previous observa- 
tions. Thus the true intensity of the mean reading 146 millims. in Experiment B is 
found by interpolating from the true intensities of the mean readings 151 and 131 found 
in Experiment A. 
From these numbers the following intensities are obtained for the undermentioned 
point on the strip in question : — 
f Philosophical Transactions, 1863, vol. cliii. p. 157. 
