I shall make a few trifling additions to M. Peclet’s clear description of his excellent mode of measuring the intensity of lights. It is, of course, presumed throughout, that the centres of the flames should be on one level; and I have found it most convenient to place the lamps on small carriages with rollers, which are guided by means of fine strips of wood nailed along the table in the directions _g_ M and _f_ M, and carrying the divided scales of centimètres. This affords the means of making any slight change in the position of the lamps so easily, as entirely to avoid the disturbance of the flame which ensues from lifting the lamp and readjusting it in another position; and will, in practice, be found very convenient when many observations are to be made. I have already said that my own experience has satisfied me that, with the aid of a good opera-glass, the central observation of the two shadows, with the opaque rod between them, is by far the best, and conducts, at once, to a result which is confirmed by the observations of two assistants who watch the shadows at the same time on opposite sides of the table, and at equal distances from them. I have found it convenient in comparing lights, to cover the table with dull black linen cloth, and to surround it with curtains of the same material, hung from slender brackets, in such a manner as to leave space for the observer to move freely round the table within them. The curtains prevent reflection from the walls of the chamber in which the experiments may be conducted, and also lessen the disturbing effects of currents of air. When a comparison of the _intensity_, and not of the _aggregate power_ of two flames, is to be made, it is necessary to adopt the precaution of inclosing the lights in opaque boxes, with slits of equal area in each, placed on the same level, and so arranged, in reference to the flames, as to be directly opposite the brightest portion of each. After what has been said, it will be almost needless to add that the _quotient of the square of the greater observed distance divided by the lesser, is the ratio of the illuminating power of the two flames_. The most convenient mode of registering observations, and that which is generally practised, is in the form of a Table like the following:-- +-------+---------------+---------------+---------------+ | | | | Illuminating | | | | | Power, | | | | Squares of | or Quotient | | | Distance. | Distances. | of Squares. | | +-------+-------+-------+-------+-------+-------+ |Trials.|Lamp A.|Lamp B.|Lamp A.|Lamp B.|Lamp A.|Lamp B.| +-------+-------+-------+-------+-------+-------+-------+ | 1 | 143 | 140 |20,449 |19,700 | 1·00 | 0·958 | | 2 | 117 | 114 |13,689 |12,996 | 1·00 | 0·949 | +-------+-------+-------+-------+-------+-------+-------+