5_th_, That the extinction of one lamp in a revolving catoptric light is not only less probable, but leads to much less serious consequences than the extinction of the single lamp in a dioptric light; because, in the first case, the evil is limited to diminishing the power of _one face_ by an _eighth part_; whilst, in the second, _the whole horizon is totally deprived of light_. The extinction of a lamp, therefore, in a dioptric light, leads to evils which may be considered _infinitely great_ in comparison with the consequences which attend the same accident in a catoptric light. ~Summary of considerations as to the fitness of the two systems for Fixed Lights.~ In comparing the fixed dioptric, and the fixed catoptric apparatus, the results may be summed up under the following heads:-- 1_st_, It is impossible, by means of any practicable combination of paraboloïdal reflectors, to distribute round the horizon a zone of light of exactly _equal intensity_; while this may be easily effected, by dioptric means, in the manner already described. In other words, the qualities required in fixed lights cannot be so fully obtained by reflectors as by refractors. 2_d_, The _average_ light produced in every azimuth by burning one gallon of oil in Argand lamps, with reflectors, is only about _one-fourth_ of that produced by burning the same quantity in the dioptric apparatus; and the annual expenditure is L.140, 3s. 8d. less for the entire dioptric light than for the catoptric light. 3_d_, The _characteristic_ appearance of the fixed reflecting light in any one azimuth would not be changed by the adoption of the dioptric method, although its increased _mean_ power would render it visible at a greater distance in almost every direction; the only exception being in the azimuths opposite the axis of each reflector, where the catoptric light has an _excess_ of power equal to about 50 _per centum_. 4_th_, From the equal distribution of the rays, the dioptric light would be observed at equal distances in every point of the horizon; an effect which cannot be fully attained by any practicable combination of paraboloïdal reflectors. 5_th_, The inconveniences arising from the uncertainty which attends the use of the mechanical lamp, are not perhaps so much felt in a fixed as in a revolving light, because the greater simplicity of the apparatus admits of easier access to it, in case of accident. 6_th_, But the extinction of a lamp in a catoptric light, leaves only one _twenty-sixth part_ of the horizon without the benefit of the light, and the chance of accident arising to vessels from it, may, therefore, be considered as incalculably less than the danger resulting from the extinction of the single lamp of the dioptric light, which deprives the whole horizon of light.