Description of Electrical Apparatus and Experiments on Mr Stevenson's Proposal to illuminate Beacons and Buoys; with Description of a New Contact-Breaker. By W. D. HART, F.R.S.S.A., Philosophical Instrument Maker, Edinburgh. *

Mr Thomas Stevenson first drew my attention to the subject of the illumination of beacons and buoys by means of electricity from the shore, connected by wires or cables to the beacon where the light was wanted, on 2d January 1866. He then gave me orders to arrange the induction coil and other apparatus to try some experiments. Our first experiment to produce the electric light for this purpose was by rapidly charging and discharging a Leyden jar with the induction machine between points of wires; and so long as we kept all the apparatus together we were very successful. We then got about half a mile of insulated wire, and tried to pass the secondary current through it, but without the slightest effect. The next thing was to get a proper cable, to ascertain the distance the secondary current. could be conveyed from the coil to the place where the light was required. A cable was supplied by Messrs Siemens of London; and after being laid in the water between Granton and the Chain Pier, we got all the apparatus arranged to pass the secondary current through it, but we did not get the slightest effect. After making a number of experiments with the cable, both in the water and out of it, we found that, however well the cable was adapted for telegraphic purposes, it was not at all suitable for our purpose. The copper conductor being too thick, it got charged, and acted as a Leyden jar, and the charge seemed to pass through the gutta percha coating to the copper sheathing outside. A cable with a thin wire, and well insulated, might answer much better; but we saw that with the best cable there

* Read before the Society, and light exhibited in action, 22d April 1867. Awarded the Society's Silver Medal and Plate, value Three Sovereigns.

was always a danger of its being pierced and destroyed by the secondary current, so we were obliged to try some other plan.

Mr Brebner suggested that we should place the coil on the beacon, along with the optical apparatus, and have the brake and batteries on the shore. This was the next plan. tried; and, after a number of experiments, we were able to transmit the primary current along a wire to the coil, and produce the desired effect. For this purpose a brake or selfacting transmitter was required, and there is one on the table which we used in our last experiments, which I shall afterwards explain. This instrument answered the purpose very well; but we found that there was a great retardation in the wire, so as to cause the coil to miss a spark or two in every five or six. I thought of another plan; and I am now happy to show to the Society a new apparatus I have constructed which completely remedies this. It is made similar to the other, but has two magnets to give it more power, and a double brake so as to send the current first in one direction, then in the other; that is to say, the one side of the brake sends a current from the zinc pole of the battery along the suspended wire, or through the cable, and the other side of the brake sends it in the reverse way. So that, by using a proper conductor and properly constructed apparatus, we can now fix on a beacon at a distance from the shore a piece of apparatus to give a light by electricity suitable for the illumination of beacons and buoys.

Some time ago Dr Strethill Wright recommended me to employ two or more coils of small intensity, and one large one of high intensity, all the positive and negative poles being united together respectively, so as to obtain a spark broader and denser, but shorter than that produced by one large coil. He considered that by this means the lightgiving power of the apparatus might be increased to any extent, and the small coils would be far less liable to destroy themselves than would the single large coil of high intensity. I agreed with him in this, but could not at that time try the experiments, as we had only one coil. Some time afterwards I saw that Dr Robinson had connected a number of coils together, and got very wonderful results. I was

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then making another coil, and, with the two coils connected together, I tried a number of experiments with very great success. I then showed Mr Stevenson what I had done, and he was quite satisfied it was much better to arrange them in this way than to make large coils, as there is always less danger of a small coil giving way in the insulation than a large one.

By the account now given you will see that the apparatus which is fixed on the beacon requires no attention, as the flash of light takes place between points or balls of platinum, or any metal not liable to corrode, fixed at the proper distance; and as they are not consumed or wasted, as in the ordinary electric light apparatus, they require no further attention. The battery and brake are placed on the shore in a house for the purpose, and are always under the control of the person in charge of them. Six or eight Bunsen cells are all that are required, and will keep up the light sixteen hours for about 2s., which is one of our longest nights. So far, then, as we have gone, things are very favourable.

As Mr Stevenson has already stated, the Messrs Stevenson were applied to by the Ballast Board of Dublin to illuminate a beacon in Valentia Harbour. To test the practicability of this, they had an apparatus fitted up in one of their experimental rooms in George Street, and the apparatus was kept working for a week, with the current passing through a wire 800 feet long, and all the time the electric apparatus wrought quite satisfactorily. The wire found to answer best was No. 4 copper wire, as in our former experiments we found a thinner wire would not convey a sufficient current to charge the coil. After dark the wires were adjusted in the focus of lighthouse apparatus, and set in front of one of the windows, and, when viewed from the Castle, the light was quite different from the other lights throughout the town, and much more brilliant, and could have been seen at a much greater distance, so that it is well adapted for beacon lights; and I have no doubt that in the course of time it will be applied to lighthouses as well as beacons and buoys.

Report of Committee.

Your Committee beg to report very favourably of Mr Hart's new Current Break. It is an ingenious application of known principles and constructions in electric science, and it does its work both steadily and effectively. They would humbly recommend it to the favourable notice of the Prize Committee.

R. M. FERGUSON, Convener.
T. STRETHILL Wright.
WILLIAM LEES.

A Method of Equalising the Illumination of the Image in Landscape Photography. By G. H. SLIGHT, C.E.*

In almost every photograph of a landscape taken with an ordinary lens and small stop, where a large angle of view is represented, there is a very decided falling off of light at the sides of the picture, from a number of causes connected with the obliquity of the pencils of rays of light which form those portions of the picture.

The most important of these causes are-First, the diminution of the apparent size of the stop (or opening through which the light has to pass to the lens), as the direction of the pencil becomes more and more oblique; second, the increased distance between the lens and the plate; third, the increased obliquity with which the pencil falls on the plate; and, fourth, the reflection from the surfaces of the lenses, which oblique pencils are subjected to. Each one of these causes destroys its share of the illumination, until in many cases the amount of light which is usefully employed in impressing an image at the sides may be less than one-half of that in the centre of the picture.

Different arrangements of lenses have different effects in this way, as it has been shown that an ordinary landscape

Read before the Society, and illustrations exhibited, on 8th April 1867 Awarded the Society's Silver Medal.

lens of the old form, which crowds the outer rays together, and gives curved marginal lines, causes less of this loss of light than the more perfect combined lens or doublet, which gives a non-distorted picture; and partly, perhaps, on this account the common distorting landscape lens has hitherto found more favour than the non-distorting kind. The amount of illumination at the sides with the common landscape lens may, however, still be only about three-fourths of that in the centre.

Several methods have been proposed for overcoming this great and acknowledged evil, of which, perhaps, the best in principle is one proposed by Mr Bow, consisting of the use of green glass for one of the convex lenses of the arrangement, so as to retard the light in the centre more than at the sides, where the lens is thinner; but none of the methods have been adopted in even the most modern of lenses; and the approach to regularity found in many photographs falls to be accounted for by over exposure in the camera, or dexterous manipulation in developing the image.

With the use of the phantascopic camera, the principle of which is that the camera revolves, and the plate moves along behind it during exposure, so as to roll down as it were the picture on the plate, perfect illumination is obtained throughout almost any angle of view, but at the expense of correct perspective, if the picture be kept flat, as it usually is, so that, except for particular purposes, it is not so useful nor so correct as the ordinary fixed camera, embracing an angle of from 50° to 70° at the utmost.

There is little doubt that if a simple means can be found of equalising the illumination of the image in using a nondistorting or doublet lens, it would be preferred to all others; and, therefore, it is to the possible improvement of the doublet arrangement that I wish to call special attention.

For what I have to allude to in the way of calculations regarding lenses, I am indebted to Mr Robert H. Bow, who, on the occasion of my bringing the subject before the Edinburgh Photographic Society last year, kindly went very minutely into the calculations required to show the effects and the application of the improvement I had suggested.