the divergence which would be caused by an increase in the size of the light--the more the divergence, the more the light is diffused and lost. We are therefore restrained, by the condition of the light and the apparatus, to a certain sized lamp. At Teignmouth, some of the revolving lights have ten lamps and reflectors, all throwing their light forward at once. But even with ten lamps and reflectors, we do not get sufficient light; and we want, therefore, a means of getting a light more intense than a candle in the space of a candle--not merely an accumulation of candle upon candle, but a concentration, into the space of a candle, of a greater amount of light; and it is here that the electric light comes to be of so much value. Let me now shew you what are the properties of that light which make it useful for light-house illumination, and which has been brought to a practical condition by the energy and constancy of Professor Holmes. I will, first of all, shew you the image of the charcoal points on the screen, and draw your attention to the spot where the light is produced. There are the coal points. The two carbons are brought within a certain distance; the electricity is being urged across by the voltaic battery, and the coal points are brought into an intense state of ignition. You will observe that the light is essentially given by the carbons. You see that one is much more luminous than the other, and that is the end which principally forms the spark. The other does not shine so much, and there is a space between the two, which, although not very luminous, is most important to the production of the light. Dr. Tyndall will help me in shewing you that a blast of wind will blow out that light--the electric light can, in fact, be blown out easier than a candle. We have the power of getting our light where we please. If I cause the electricity to pass between carbon and mercury, I get a most intense and beautiful light--most of it being given off from the portion of the mercury between the liquid and the solid pole. I can shew you that the light is sometimes produced by the vapour between the two poles better, if I take silver, than when I use mercury. Here is the carbon pole, there is the silver, and there is the beautiful green light, which comes from the intervening portions. Now, that light is more easily blown out than the common lamp, the slightest puff of wind being sufficient to extinguish it, as you will see if Dr. Tyndall breathes upon it.