At the meetings of the Society on February 1 and 15, 1875, I exhibited a number of photographs which had been executed for me in my laboratory by Mr A. Matheson, one of my students who has acquired great skill in working with very sensitive films of collodion. My object in advising him to undertake this work was to discover, if possible, the cause of the peculiar zig-zag form which electric sparks, and specially those of a Holtz machine, always show in ordinary air. At the desire of the Council, a selection of a few of the more interesting of these photographs has been printed (by Mr Dallas, F.R.S.E.) for the Transactions from the original glass negatives, by what is commonly called the carbon process.