274 



Nature and Optical Efficiency of the Mirrors 



[April 



tlie lower hemisphere. In the same manner, of two other simihu- divi- 

 sions of it, formed by a plane perpendicular to the former, (whirh is 

 supposed to be horizontal) as the front and rear hemispheres, as regards 

 tile situation of the light in reference to a reflector or any particular 

 object. The rays proceeding ilirectly from a luininous body to the eye, 

 and thus visible, may be termed the original or direct light, in contra- 

 distinction to that visible after reJlectio?i from any bright surface. 



Under mos>t of the circumstances in which light is usually required, 

 as for instance, in illuminating apartments &c. the whole of the rays of 

 direct light, corresponding to every part of the above sphere, are of 

 equal value, those proceeding in an upward direction serving to illumi- 

 nate the ceiling and upper parts of the room, and those proceeding in a 

 direction from the eye, being reflected from other objects and render- 

 ing them visible ; the intention being in this case, to afford as much 

 light as possible to the various objects placed in different situations 

 round the luminous body, and not particularly to throw a large quantity 

 towards the eye; as it is not the I'^ght, but the objects, which it is most 

 desirable should be seen. 



In a Liglit-house, hovv-ever, the case is just the reverse, for there are 

 here no objects whatever to be illuminated, the great point to be attain- 

 ed being to throw as much light as possible to the observer. Now as: 

 it is evident that in respect to a luminous body placed in a Light-house 

 the eye can never be situated in any part of the zipj}er\Vc\}J of the sphere 

 of rays above alluded to, since the line connecting the light and tlie ho- 

 rizon would be barely even horizontal, it follows that the whole of those 

 fays must be entirely lost. If the Light-house from being insulated be 

 required to illum.inate the circumference of the horizon all round, nearly 

 all the remaining, or lower half of the sphere of rays, will be elective, as 

 those which fall below the horizon line Avill be visible from points nearer 

 to the light, and consequently forming an angle of depression with it ; 

 and the only rays of the lower hemisphere lost will be those intercepted 

 l>y the brick work of the building itself. This will I hope be clearly 

 understood by a reference to Fig. 2 which represents a vertical section 

 of a Light-house situated as above supposed, and in which the unser- 

 viceable rays, or those lost in consequence of their emanating in di- 

 rections above the level of the horizon, are comprised within the semi- 

 circle ah c (which is the section of the upper hemisphere before spoken 

 of) while the serviceable rays, or those which are visible from points at 



different distances from the building, are included wiihin the lower 

 scmiciiele d e f. 



In many instances, however, illumination m^iy be required to extend 



