for the purposes of Illumination in Lighthouses. 49 



This remark applies particularly to the back portion MCN, Fig. 6. 

 Plate III. of the reflector, which includes a whole hemisphere of 

 the rays which radiate from KL ; and as all the rays included 

 between LA and LB are not incident upon the reflector, its 

 main effect must be produced by the action of the zone corre- 

 sponding to the rays between MLA and LNB, which will ren- 

 der the column most luminous near its circumference, and least 

 luminous along its axis. 



The reader who has followed us in these observations, must 

 have anticipated the conclusion, that a parabolic reflector shaped 

 by the hammer, and furnished with an argand-burner, whose 

 flame is only three or four inches from the back of the reflector, 

 cannot possibly afford a parallel and dense beam of light, capable 

 of penetrating space, and forcing its way through the haze even 

 of an ordinary atmosphere. That this conclusion is well found- 

 ed, may be readily proved by examining the distribution and in- 

 tensity of the light in different sections of the reflected beam, 

 taken at considerable distances. In one of the best reflectors 

 which I have seen, I observed, even at the distance of twenty 

 feet from it, a large dark spot on its surface. This opening, or 

 space destitute of light, must have been so enormously great at 

 the distance of five or six miles, as to diminish very considerably 

 its penetrating power. 



But, independent of the dispersion of the light by imperfect 

 reflexion, and its deviation from the axis of the parallel beam, 

 there is a great portion of the light lost by the use of hammered 

 reflectors. The loss of light arises from two causes, namely, the 

 absorption of the light by the metallic surface, and the loss of 

 light by the collision of the rays at their points of intersection. 

 All metallic surfaces, even when highly polished and perfectly 

 smooth, absorb on an average one-half of the light which falls 

 upon them ; but while the hammered reflectors are peculiarly 

 liable to that imperfection, the convergency of the pencils which 

 they reflect, occasions a loss of light almost equally great. Cap- 



VOL. XI. PART I. G 



