916 PEOFESSOE BWSEN AM) DE. H. E. EOSCOE’S PHOTO-CHE>nCAL EESEAECHES. 
that the illumination of the sunshine is equal to that from the daylight when the 
distance of the sun from the zenith is 71° 12'. 
These daily phases of equal chemical intensity can be determined by allowing the 
diffuse daylight and the direct sunlight to fall separately on two pieces of the same 
photographic paper, and by observing the time at which the same degree of colour is 
brought about on both pieces. From the coiTespondence of these obseiwed times with 
those calculated according to the foregoing method, a good idea can be formed of the 
degree of reliability which our experiments, and the theory based thereon, warrant. 
On February 21st and 22nd, and March 7th and 11th, 1859, the sky was perfectly 
cloudless from sunrise to sunset, and hence these days were well-adapted for the pur- 
pose of the experiments. The observations were made in a darkened space under the 
roof of the Heidelberg laboratory by two persons, one of whom, outside the roof, allowed 
the whole diffuse light of day, excluding the direct sunshine, to act upon the prepared 
paper, whilst the other observer allowed the direct sun’s rays to fall inside the darkened 
space for an equal length of time perpendicularly upon another portion of the same 
paper. The sunlight passed through a hole in the roof, on to a blackened box with a 
circular opening of 1 inch in diameter. In order to preserve the paper always at right 
angles to the incident rays, it was fixed on a piece of board held in the hand, upon 
which an upright pin was fastened, and the board was so placed that the shadow of the 
pin on the paper was never thrown to one side or the other. 
Allowing for the actual height of the barometer observed. O'" -76 4, the zenith-distance 
of the sun, at which the chemical illumination from the direct sunlight is equal to that 
from diffuse light, is found to be 71° 4'. The sun reached this point on February 21, 
1859, at 9^' 7“ a.m., and at 2^ 63®" p.m. The experiments on this day began at IH 30“ 
A.M., when the blackening effected by the sunlight was much more considerable than 
that produced by daylight ; at a later hour this difference between the actions dimi- 
nished ; at 3*' 1“ scarcely any difference could be observed, and at 3*^ 16“ the point of 
equal colour was passed, as the paper exposed to the sunlight was at this time consider- 
ably less coloured than that exposed to diffuse daylight. The phase required lies there- 
fore between 3*^ 1“ and 3^^ 16'“ ; according to calculation, it should have occuiTed at 
2*^ 53“. Another determination, made on the following day, showed that the pheno- 
menon occurred in the afternoon between 3** 23“ and 3*' 39‘", instead of at 2*^ Sd*"; and 
in the morning between 8^ 30‘“ and 8^ IS*", instead of at 9** 4'“. On March 7, 1859, under 
a barometric pressure of 0'“‘762, the phase occurred in the afternoon between 4** 17“ 
and 4’’ 27“, when, according to calculation, it should have occurred at 3’* 33“. On March 
11, 1859, the sky was intensely blue; the phase of equal colour appeai'ed, under a 
pressure of 0‘“'764, in the morning at 7’* 42“ instead of 8'-' 22™. From these experi- 
ments, it is not only seen that these phases which the theory requires actually occur, 
but also that the agreement between the observed and calculated time of appearance of 
these phases is very close. The point at which the phenomenon is observed in the 
morning is indeed about forty -five minutes earlier than the calculated point, whilst in 
