456 SEC. 12. APPLIED MECHANICS. 



2nd. The central lens (or dioptric) comprehends a vertical angle of 76 

 degrees, whereas, in the old sections, this angle was of about 60 degrees only ; 

 its elevation is thus increased from 0'85 m to M0 m . This advantage is thus 

 obtained, that the luminous rays meet the last dioptric element at the same 

 angle as the first catadioptric ring, and suffer no more loss of reflection upon 

 the one than upon the other. 



3rd. The section commonly used in apparatus of the second class had been 

 calculated for a three-wick lamp burner of 0'074 m diameter. With a five-wick 

 burner of 1*11 O m diameter, the inferior elements of the dioptric lens and the 

 lower catadioptric rings, constructed after this old section, emit rays that are 

 no longer in the proper direction, because the portion of light which the base 

 of the burner leaves visible becomes perceptibly nearer to the lens than in the 

 case of a three -wick burner. To lessen this defect, a graduated shape was given 

 to the burner, by placing each wick 0'002 m below the one preceding it on the 

 side of the centre. This arrangement reduces neither the regularity nor the 

 intenseness of the light, and the part of that light, visible from each of the 

 lewer lenticular elements, becomes somewhat increased. Moreover, these 

 lower elements have been calculated by determining for each of them a par- 

 ticular focus taken on the brightest line of the apparent part of the light, 

 instead of on the axis itself of the lamp. Similar arrangements might be 

 advantageously adopted in many cases. 



4th. The central lens and the lower rings are included in the same frame, 

 the upper rings are set in a second frame, separated from the first by a metal 

 cross-beam. In the annular lenses, this cross-beam takes the shape of the 

 arc of a circle having, like the rings, its centre on the optical axis ; the result 

 is that the rings can remain intact, instead of having to be cut, as was the case 

 until now. 



5th. The lamp, placed at the focus of the lens exhibited, shows special 

 arrangements, due to M. Denechaux. 



Thus, the skin pockets or valvulffi, and the leathern valves, which are 

 sometimes the cause of disorder, are replaced by ordinary pistons and metal 

 valves. This system has produced good results in experiments made at the 

 depot, but it has not yet received practical sanction. 



The lamp with five wicks for burning mineral oil has an intenseness of 

 36 carcel burners, the fixed light apparatus produces an intenseness of 640 

 burners, and the annular lenses produce an effulgence of more than 5,000 

 burners. 



No. 13. Apparatus for electric revolving light, constructed by Messrs. 

 Sautter, Limonnier, & Co., 1876. 



This instrument is intended to produce, by electric light, a light revolving at 

 intervals of 30 seconds. It includes a fixed light apparatus 50 m diameter, 

 lighting the three-fourths of the horizon, around which revolves, in eight 

 minutes, a tambour of 0'62 m diameter, and composed of 16 vertical, lenticular 

 elements. 



In the section of the fixed light apparatus the central dioptric part fills 

 vertically an angle of 76 degrees, which is greater than in the old sections. 

 This arrangement is adopted in order that the luminous ray may meet the 

 last dioptric element at the same angle as the first catadioptric ring, and should 

 suffer no more loss by reflection upon the one than upon the other. The 

 apparatus having to be fixed on an elevated point, the section of the several 

 parts, except that of the two lowest catadioptric rings, has been calculated so 

 as to throw the focal line of the emergent rays, 30 minutes below the horizontal 

 line ; in the calculation of the two lowest rings, this angle is increased by 

 three degrees for the last but one, and by five degrees for the last, so that the 

 lighthouse may remain visible at a short distance, that is, by a navigator 

 placed below the divergent cone emitted by the rest of the apparatus. 



