274 Thomas Stevenson on the 



from using several parabolic reflectors, but these at first are 

 costly, and, what is worse, the annual expense of maintaining 

 so many independent burners is very great. 



In order to supply this deficiency, I have already, in my 

 description of the Holophotal system of illumination, published 

 in the Transactions of the Royal Scottish Society of Arts 

 for 1850, proposed a method of distributing all the diverging 

 rays which proceed from a flame over 90° of the horizon. It 

 is, however, often desirable to illuminate both larger and 

 smaller arcs than the quadrant, and I now proceed to explain 

 a simple method of attaining this end. 



The whole diverging sphere of rays proceeding from the 

 lamp is, in the first place, collected into one beam of parallel 

 rays by means of a holophotal apparatus, and the rays so 

 united are afterwards subjected to a second action, by' which 

 any desired amount of divergence may be produced. 



In figure 1 (Plate IV.), a represents a parabolic conoid, 

 truncated at its parameter ; b is a hemispherical mirror ; and I, 

 a lens which, when placed at its proper focal distance from 

 the flame, subtends the same angle from it as the outer lips of 

 the paraboloid. The hemispherical reflector occupies the place 

 of the parabolic conoid which has been cut off behind the para- 

 meter, and the flame is at once in the centre of the hemisphe- 

 rical mirror and in the common focus of the lens and paraboloid. 

 Such an arrangement of optical agents constitutes a holopho- 

 tal apparatus ; for if we suppose the whole sphere of rays 

 emanating from the flame to be divided into two portions, 

 namely, the hemisphere of front rays and the hemisphere of 

 back rays, it is obvious that part of the exterior or front hemi- 

 sphere will be intercepted by the lens, and made parallel by 

 its action, while the remainder will be intercepted and ren- 

 dered parallel by the paraboloid. The rays forming the 

 posterior hemisphere, and which fall upon the hemispherical 

 reflector, will be sent back through the focus in the same lines, 

 but in opposite directions to those in which they came, whence, 

 passing onwards, they will be in part refracted in a parallel 

 direction by the lens, and the rest will be reflected in a parallel 

 direction by the paraboloid. The back rays thus finally emerge 

 horizontally in union with the rays from the anterior hemisphere. 



