for the purposes of Illumination in Lighthouses. 39 



having all their foci coincident at F ; though it. will, perhaps, be 

 more convenient to have the posterior part MN without lenses, 

 and occupied by a mirror of nearly the same radius FA as the 

 sphere. The object of this mirror, is to throw back upon the 

 object at F the light that passes by it without producing any ef- 

 fect. Each of the lenses, except the lens A, is furnished with a 

 plane glass mirror, which may be either fixed to the general 

 frame of the sphere, or placed upon a separate stand. When 

 this combination is completed, the sphere is exposed to the sun, 

 so that its rays may fall at right angles upon the lens A, which 

 will, of course, concentrate them at F, and produce a pretty in- 

 tense heat. The plane mirror PQ, when properly adjusted, will 

 reflect the sun's light perpendicularly upon the lens B, by which 

 it will be refracted accurately to the focus F, and produce a de- 

 gree of heat fully one-half of what was produced by the direct 

 refracted rays of the sun through the lens A. A similar effect 

 will be produced by the mirror RS and lens D, the mirror TU 

 and lens C, the mirror VW and lens E, and all the other mirrors 

 and lenses which are not seen in the section. The effect may be 

 still farther increased by the addition of a large lens at XX. As 

 the angle which the surface of each mirror forms with the axis 

 of its corresponding lens, is a constant quantity, the mirrors may 

 be all fixed to the general frame of the sphere, and therefore the 

 only adjustment which the instrument will require, is to keep 

 the axis of the lens A parallel to the direction of the solar rays. 



" In order to estimate the advantages of this construction, let 

 us compare its effects with those of a solid lens, which exposes 

 the same area of glass to the incident rays. 



" 1. In the burning sphere, almost the only diminution of 

 light is that which arises from reflection by the plane mirrors, 

 and which may be estimated pretty accurately at one-half of the 

 incident light ; but this loss can be amply compensated by add- 

 ing a few more lenses. 



