KEGISTEATION OF THE CHEMICAL ACTION OE TOTAL DAYLIGHT. 
617 
IV. Concerning the accuracy and trustworthiness of the method. 
The most satisfactory mode of testing the reliability and accuracy of the method of 
measurement just described, is to compare the results of two series of independent 
determinations of the chemical action of daylight, made simultaneously at the same 
spot with the present arrangement and with the pendulum photometer, according to 
the method described in the last memoir, upon which the present mode of measure- 
ment is founded. For the purpose of making these comparisons, the strips of standard 
photographic paper placed in the pendulum apparatus (see fig. 1 of last memoir) and 
the pieces of the same material placed on the insolation-band in the exposing slide 
(fig. 3, A) were simultaneously insolated, each for a known length of time, both instru- 
ments being placed near one another in a position (on the roof of the laboratory of 
Owens College, Manchester) having a tolerably free horizon. If the varying daily 
intensities thus measured by the two methods are found to agree, we may conclude 
that the unavoidable experimental errors arising from graduation, exposure, and reading 
are not of sufficient magnitude materially to affect the accuracy of the measurement. 
The intensity with the pendulum photometer was determined exactly as described on 
pp. 158 & 159 of the above-cited memoir ; the time of exposure and the number of 
vibrations were noted, the position at which the strip possessed a shade equal to that of 
the normal tint was then read off, and the corresponding intensity obtained by dividing 
the number found in Table II. of the above memoir by the number of the vibrations. 
The intensity, according to the new method, was obtained by insolating the standard 
paper in the exposing slide (fig. 3, A) for a known number of seconds, and then reading 
off, by means of the arrangement shown in fig. 6, the position in millimetres on the 
calibrated strip equal in shade to the exposed paper. The number found in the second 
column of the Intensity Table, of the strip opposite to this position, when divided by the 
time of exposure in seconds, gives the required intensity. In this way comparisons of 
the working of the two modes of measurement have been made during four different 
days. On each of these days a large number of simultaneous observations were made, 
and on some of them two or more determinations were made with each instrument 
immediately succeeding each other. An examination of the following Tables, giving 
the results of these observations, shows that the agreement between the intensities 
as obtained by the two methods is as close as can be expected. 
