| 1250·0| 150·67 |1211·90|| V.|1250·00| 452·23| 1184·20| | II. | 1250·0| 228·19 |1208·80|| VI.|1250·00| 453·07| 1185·20| | III. | 1250·0| 305·10 |1203·35|| VII.|1250·00| 374·60| 1196·00| | IV. | 1250·0| 381·00 |1195·30|| VIII.|1250·00| 294·35| 1203·80| +------+-------+---------+-------++------+-------+--------+--------+ ~Power of Dioptric Instruments.~ The effect of an annular lens, in combination with the great lamp, may be estimated at moderate distances to be nearly equal to that of 3000 Argand flames of about an inch diameter; that of a cylindric refractor at about 250; and that of a curved mirror may perhaps on an average be assumed at about 10 Argand flames. ~Orders of the French Lights.~ The dioptric lights used in France are divided into four orders, in relation to their power and range; but in regard to their characteristic appearances, this division does not apply, as, in each of the orders, lights of identically the same character may be found, differing only in the distance at which they can be seen, and in the expense of their maintenance. The four orders may be briefly described as follows:-- 1_st_, Lights of the first order having an interior radius or focal distance of 36·22 inches (92^{cm.}), and lighted by a lamp of four concentric wicks, consuming 570 gallons of oil per annum. 2_d_, Lights of the second order having an interior radius of 27·55 inches (70^{cm.}), lighted by a lamp of three concentric wicks, consuming 384 gallons of oil per annum. 3_d_, Lights of the third order, lighted by a lamp of two concentric wicks, consuming 183 gallons of oil per annum. The instruments used in those lights are of two kinds, one having a focal distance of 19·68 inches (50^{cm.}), and the other of 9·84 inches (25^{cm.}). 4_th_, Lights of the fourth order, or harbour-lights, having an internal radius of 5·9 inches (15^{cm.}), and lighted by a lamp of one wick, or Argand burner, consuming 48 gallons of oil per annum. This apparatus is, as already noticed, now more generally used of a larger scale, having a focal distance of 9·84 inches (25^{cm.}), and a lamp of two concentric wicks, consuming about 130 gallons of oil per annum.[73] [73] An apparatus of 0^{m.}·185^{cm.} was recently added to the list of French lights under the name of the _Fifth_ order; while that of 25^{cm.} radius has been called the _Fourth_, and that of 15^{cm.} radius is styled the _Sixth_ order. Those minute subdivisions I consider to be unnecessary. ~Distinctions of the Dioptric Lights.~