TRANSACTIONS OF THE SECTIONS. 13 
than the rays near the circumference, the mean refrangibility of the photogenic rays 
alters according to the parts of the object-glass which operate photogenically; hence 
when the yellow rays are abundant the mean refrangibility of the photogenic rays is 
proportionally increased, because the centre does not contribute to the photogenic 
image. The colour of the glass of the lenses has also an influence in the concen- 
tration of yellow rays, their chromatic correction another; hence the anomaly of 
the same light affecting differently the separation of various object-glasses. ‘The 
dynactinometer can also be applied for measuring the intensity of the photogenic 
light at any moment ; and as a regular motion is required for this object, the move- 
ment is given to the black disc by means of clockwork. 
Report of a Committee appointed to examine the Effects produced by Light- 
ning on a Tree near Edinburgh. By Professor ῬΗ 1,118, £.A.S. 
The tree in question stands in the grounds of Mr. Wauchope, at Edmonstone, 
about four miles from Edinburgh, on the Dalkeith road. The surface slopes gently 
to the north; the substrata are part of the coal formation, and contain at a small 
depth an abundance of the rich ‘black band’ ironstone. The locality appears re- 
markably liable to lightning strokes; several other trees having been destroyed there 
since 1834. _ 
The tree examined by the Commitee was struck on the 11th of June 1849, on a 
still sultry day. It is an oak-tree ; it stood in rather a clear space, the surrounding 
trees being chestnut, elm, &c. It was a large tree (14 feet in girth) ; but there were 
others as high, and of rather greater diameter. When struck it was full of sap. 
The mechanical effects of the lightning were violent. The main trunk of the tree, 
which appears to have stood about 12 feet high before sending off branches, was 
rent from top to bottom; some of the branches were broken off; all were thrown 
down and implicated together, and for some distance upward fissured and twisted : 
some of the roots were split a yard or more from the stem. A large mass from the 
northern side of the tree was driven out, and carried through the air 127 feet, in the 
direction of the magnetic meridian, to N.N.W. Its weight was 21 ον. 
The main stem was entirely denuded of athe bark, which was scattered widely 
around, but most abundantly in a direction opposite to that in which the log of 
wood was conveyed. Shreds of wood were carried to the north-west and left hang- 
ing in the trees. 
What remained standing of the stem, as well as the parts which had been dis- 
placed, was cleft into wedges, by vertical radiating fissures parallel to the lamine of 
medullary rays; and these wedges were again cleft by other vertical fissures con- 
centric to the axis of the tree, and coinciding with the annual bands of large vertical 
vessels, which are conspicuous in cross sections of the oak. Where these cleavages 
produced the fullest effect, the wood was divided into long slender prismatic shreds 
like lucifer matches. These split masses were much twisted. 
For all these phenomena a simple mechanical cause appears sufficient, viz. an 
internal expansion and bursting of the main stem of the tree, along the surfaces 
which, by the structure of the tree, admitted of the most easy separation, and con- 
tained at the time abundance of liquid sap, capable of assuming the form and force 
of elastic vapour. Hence, in the first place, the destruction of the main stem by 
explosion ; the projection of the bark and woody fragments, and the minute and 
regular cleavage of the fibres. The stem being destroyed as a support, the branches 
fell in ruinous aggregation round it. 
It appears that a laburnum-tree, situated about 12 yards to the east, had been 
twice struck by lightning, first (I believe) in 1834, and again in 1844. It was split, 
. but not barked. 
An elm, situated about 100 yards to the north, was struck, and in like manner 
split, but not barked. These differences may perhaps be due to the difference of 
structure in the wood; but in all cases, before attempting to explain the phenomena 
observed as the effects of lightning, it is desirable to be informed of the time of 
year when the trees were struck. 
The precise points of entrance and exit of the lightning cannot be stated in the 
case now before us. A small quantity of black powder was found in the fissured 
