M. Servier has examined the question of lighting a street 20 mètres wide and one or more kilomètres long, with the condition that the illumination of the ground shall present a mean determinate quantity of light per square mètre, or a given intensity at the darkest points. Some interesting results are thus obtained. Thus, by substituting for the 14-Carcel (133-candle) lamps in the Rue du Quatre Septembre, burners of 50-Carcel (475-candle) power, with the condition of giving the same intensity of 0·5 Carcel (4·75 candles) to the darkest points, a quantity of light more considerable than before will be required. That is, a greater number of Carcels (3000 as against 1848 per kilomètre in length) will be necessary in the larger burners than were required in the original smaller lamps. It is therefore imperative, in order that the lighting shall be equally economical, that the unit of intensity--the Carcel or candle power--shall be less costly in a lamp of 50 Carcel (or 475-candle power) than in the smaller lamps. By fixing lamps of 50-Carcel (475-candle) power in the centre of the street, instead of along the side walks, maintaining the condition of giving the light of 0·5 Carcel (4·75 candles) in the darkest parts of the thoroughfares, it is found that the pillars must be 8 mètres high and 20 mètres apart. The best-lighted part of the road would then have the intensity of 1 Carcel (9·5 candles), and would therefore be only twice as brilliantly lighted as the darkest corner; the mean quantity light per square mètre would be 755 _décicarcel-cubes_. Lastly, the same method of lighting has been applied to the “ordinary,” as distinguished from the “luxurious” lighting of the public thoroughfares, assumed to be 20 mètres wide, giving a light of 0·05 Carcel (0·475 candle) at the darkest points. With ordinary street burners consuming 200 litres (7 cubic feet) of gas per hour, and giving 1·72-Carcel (16·34-candle) power, it is found that the lamps should be 18 mètres (20 yards nearly) apart, the burners being 3 mètres (9 feet 10 inches) high. With burners of 14-Carcel (133-candle) power placed at the height of 3·20 mètres (10 feet 6 inches), the lamp-pillars would be 106 mètres (115 yards) apart. Or with lamps of 50-Carcel (475-candle) power placed at a height of 8 mètres (26·24 feet), the distance between the pillars may be increased to 270 mètres (494 yards).