368 
ME. WAREEN DE LA EUE ON THE 
conceived when it is stated that 2' on the sun’s limb do not occupy more than the space 
of T^o ofh of an inch on the photographs obtained with the Kew heliograph. 
At the moment of taking the photographs, the collodion was in a soft and moist con- 
dition, but subsequently, when the measurements were made, it had become dry. It 
became, therefore, not only a matter of interest, but of fundamental importance, to ascer- 
tain whether there had been any contraction of the collodion while drying, and, if so, 
whether the contraction had been uniform. Much care and attention were necessary 
in order to determine this point. By observing the positions of specks on the glass in 
respect of markings on a photograph while wet, it could be seen whether they retained 
their relative positions when the collodion had dried. The result, however, proved that 
there was no appreciable contraction, except in thickness, and that the collodion film 
did not become distorted, provided the rims of the glass plate had been well ground. I 
cannot show this more strikingly than by citing the measured radius of the sun on two 
photographs, namely. Nos. 6 and 45, and the measurement of the angles between the 
position-wires depicted on them. The radius of the sun No. 6 was found to be 1906-5, 
that of No. 45 1906 0 thousandths of an inch. 
No. 6 . . 
No. 45 . . 
Diff. 6—45 
Angle between 
IV. and I. 
89 59-3 
90 2 
Angle between 
1. and II. 
89 52 
89 53 
Angle between 
11. and III. 
90 8-7 
90 6 
Angle between 
III. and IV. 
0 / 
90 0 
89 58-5 
-2-7 
- 1 
+ 2-7 , 
+ 1-5 
1 
These differences, which are extremely small, do not exceed those obtained in mea- 
suring at different times the same photograph, and depend somewhat on the judgment 
exercised in causing the images of the position-wires to be bisected exactly by the wire 
of the microscope. 
Photographs of the various phases of the partial eclipse, either pre^dous to or after 
totality, exhibit a very curious phenomenon. The concave edge of the sun in immediate 
contiguity with the moon’s limb, appears brighter than the other neighbouring parts of 
the crescent, while the convex limb of the sun bordered by the dark background of the 
sky, does not appear at all brighter than its proximate parts. This brightening of that 
part of the sun’s disk which borders on the moon’s limb, extends only for the space of a 
narrow line beyond the latter, but is remarkably conspicuous. As it cannot be accounted 
for by assuming the existence of a lunar atmosphere, it naturally excited a desire to 
trace out its cause. The Astronomer Royal, to whom I pointed out the fact, ascribed it 
to the effect of contrast, and I have subjected this hypothesis to the test of experiment 
in the following manner; — Having made some photographic prints of the sun’s crescent 
on paper, which showed the appearance in a striking manner, I cut out about half of 
the crescent with sharp scissors, in such a way that the visible surface of the sun might 
be lifted up like a tongue, and replaced in its normal position within the background at 
pleasure ; on smoothing the part so cut out, and causing it to occupy its original place. 
