610 
PROFESSOR ROSCOE ON A METHOD OF METEOROLOGICAL 
(3) Experiments showing alteration and subsequent permanency of Tint. 
Sheet No. 5, prepared March 10, 1864. 
Intensity. 
Intensity. 
Intensity. 
Intensity. 
Intensity. 
Intensity. 
1st Reading, 
2nd Reading, 
3rd Reading, 
4th Reading, 
5th Reading, 
6th Reading, 
Mar. 12, 1864. 
Mar. 21, 1864. 
Apr. 27, 1864. 
May 11, 1864. 
June 3, 1864. 
July 18, 1864. 
Strip A, exposed to sunlight... 
2-08 
213 
1*93 
1-99 
2-03 
1*89 
Strip B, in the dark 
2-10 
2-13 
1*93 
1-93 
1*89 
1-89 
Sheet No. 6, 
prepared March 10, 1864. 
Strip A, exposed to sunlight... 
2-23 
2*23 
2-13 
2-15 
2-15 
2-10 
Strip B, kept in the dark 
2-23 
2*23 
1-99 
2-01 
2-08 
1-97 
Sheet No. 7, prepared March 10 
CO 
3 
Strip A, exposed to sunlight... 
2-35 
2-42 
2-08 
2-18 
2-13 
2-01 
Strip B, kept in the dark 
2-35 
2*54 
2-01 
2-03 
2-08 
2-03 
The above numbers show that, after the standard fixed strips have been prepared for 
about two months, the tints remain constant both when the paper is exposed to light 
and when it is kept in the dark. The small differences seen in some instances arise 
from unavoidable experimental errors of various kinds. 
II. Graduation of the fixed Strips in terms of the Standard Pendulum Strips. 
The value of the proposed method of measurement entirely depends upon the possi- 
bility of accurately determining the intensities of the various shades of the fixed strips 
in terms of the known intensities of the standard strips prepared in the pendulum pho- 
tometer. 
Two modes of effecting this graduation, and of comparing the accuracy of the gra- 
duation of one strip with that of another, were employed. 
The first of these methods consists in determining by direct comparison the points on 
the fixed strip having equal intensities to points on the pendulum strip. For this 
purpose the position of the standard tint upon the pendulum strip was first observed ; 
circular pieces of this strip, situated 20 millims. apart, were then stamped out with a 
punch 5 millims. in diameter, and half of each circle pasted on to the wooden reading 
block (fig. 4 of the last memoir), so that the centre of the paper circle came into the 
centre of the hole. The readings were conducted in the way described on p. 159 of the 
last memoir, every comparison being made independently ten times by each of two 
observers, and the mean reading taken as the result, whilst several pendulum strips were 
used for the graduation of one fixed strip. The following may serve as an example of 
the first method of graduation. Four pendulum strips were employed for the graduation 
of the fixed strip A. 
