56o 



NATURE 



building is yet quite finished and ready for occupation ; a 

 good deal of work has yet to be done to the internal 

 walls, the doors and windows, flooring, &c., besides the 

 scientific equipment of the building. Therefore the 

 announcement by the Alpine Club of Catania that the 



[April 14, 1 88 1 



building would be ready for inauguration at the meeting 

 of the Alpine Congress in Catania next September 

 was premature. The Observatory will not really be ready 

 to be opened til! 1882. The difficulties that have had to 

 be contended with can only be comprehended by those 



^\ho have visited the place; all the materials have to be is finished and the construction of the meteorological 



conveyed 3000 metres above the level of the sea, and that apparatus is being proceeded with. The Ministers of 



aunng only three months in the year. So that even if Agriculture and Public Instruction are doing their best to 



not ready till 1882, the work maybe said to have been provide the f bservatory, bv 1882. whh a director and 



rapidly accomphshed. The mounting of the equatorial a staff both of astronomers and meteorologists. 



MODE OF MASKING OR CUTTING OFF 

 SHARPLY THE LIGHT FROM REVOLV- 

 ING APPARATUS ON ANY DESIRED 

 COMPASS -BEARING BY MEANS OF A 

 RECIPROCATING SCREEjV 



^^\TNG to the optical properties of the lens employed 

 '^^ in revolving lights, a formidable element of difficulty 

 comes in the way of eftccting a sharp cut-off" on a par- 

 ticular bearing ; for the direction of the axis of the beam of 

 light which is projected by the lens is being continually 

 changed in the horizontal plane bv the revolution of the 

 frame on which it is fixed. So long as the axis of this 

 beam of rays points outside of the line of obscuration 

 the light will not of course encroach on the danger arc, 

 unless to a small extent, when the axis is nearly on the 

 line of cut-off due to the ex-focal rays proceeding from 

 the outer edges of the flame. The'light however will 

 begin to be diminished in power from a bearing varying 

 from 12° to 21° outside of the line of cut-oft" dependent 

 on the size of the lens, the light on the line of cut-oft" 



being diminished to the power of one-half. But when 

 the axis crosses that line, then as the rays which come 

 from that part of the apparatus which is still outside of 

 the darkened panes of the lantern is not intercepted by 

 them, the light will begin to be seen within the arc of 

 danger, and as the apparatus goes on revolving the axis \f'\\\ 

 at last point from about 12^ to 21° within the danger-arc, 

 according to the breadth of the lens which is employed. 

 Owing to this peculiarity of a revolving light the difficulty 

 of confining the flashes within any required arc of the 

 horizon by means ofyf-jvv/ screens is in fact an insur- 

 mountable one. 



The mode which 1 have to suggest is as follows : — In 

 front of the revolving apparatus and on the safety side 

 of the danger arc, let a light canvas or metallic screen 

 be constructed for running on rollers on a slightly inclined 

 rail or circular path close to the apparatus. If now a small 

 projecting rod or snug be fixed to the side of each lens 

 it will in revolving be brought against the edge of the 

 screen, and will gradually press the screen before it up 

 the inclined plane at the same rate of motion backwards 



