38 REPORT — 1871. 



of illumination for lighthouses are generally admitted. There are, however, many 

 cases, such as harbour-lights and ship-lights, where the expense of construction 

 becomes a barrier to the employment of refracting apparatus. 



In order to reduce the expense, it occurred to the author that it would be de- 

 sirable to revive the old form of miiTor, consisting of facets of ordinary silvered 

 glass. Instead of making them small and with plane surfaces, the size may be 

 much increased ; and they may be bent or groimd and polished on both faces to 

 curves osculating the parabola, ellipse, or whatever form may be required. If the 

 edges of these facets wei'e fixed together by Canada balsam (a substance which has 

 nearly the same index of refraction as plate-glass), the large loss of light which 

 takes place at tlie edges of each facet in the old reflectors will be in great measure 

 saved. There will not, as formerly, be any refraction of the rays in passing through 

 the edges, and thus the whole will become practically monodioptric ; or, in other 

 words, will be optically nearly the same as if the paraboloid had been made of one 

 whole sheet of glass, while the advantage due to accurately cm-ved sm-faces, in- 

 stead of plane surfaces, will be secured. It woiild be a further improvement to 

 select dinerent points in the flame for the foci of the different facets, so as to 

 secure the useful destination of more of the rays. Besides, by grinding each facet 

 to different vertical and horizontal curves, the light maj' be condensed or diverged 

 by means of a single agent ; and the same result may be effected with different 

 totally reflecting plates of flint or other glass cemented to lighthouse prisms with 

 Canada balsam, so as to form composite prisms. When coloured lights are wanted, 

 the facets would consist of glass tinted to the required hue, so as to render stained 

 muffles or chimneys unnecessary. The economy of the proposed method of con- 

 struction will render it peculiarly applicable to harbour-lights and ship-lights. 



The author exhibited a paraboloidal reflector constructed on the method to 

 which he refen-ed. The facets were successfullj^ constructed by Messrs. Chance, 

 of Birmingham. The pieces of glass having been first bent upon a mould, were 

 afterwards ground by rubbing-surfaces worked by machinery of the same kind as 

 is employed for dioptric apparatus. The facets were afterwards silvered by the 

 patent process of ^Messrs. Pratt and Co., of St. Helens, Lancashire, who inform the 

 author that so long as the paint is not removed from the back of the silvering its 

 reflecting power will remain unaltered. 



Differejitial metallic Mirror and Holopliote. — The same construction may also be 

 adopted, as the author has already hinted, for producing a differential holopliote 

 which will, Inj means of sinr/le optical ar/ents, collect, with uniform density in azimuth, 

 the whole spihere of diverc/inff rays into any r/iren cylindric sector. For such a piu'- 

 pose each facet must, in the vertical plane where no divergence is wanted, be 

 groimd to a parabolic profile, while in the horizontal it must be of such hA^per- 

 bolic, elliptic, or other cun-e as wiU give the required horizontal divergence with- 

 out interference with the apparatus for the central cone of rays, which wiU be 

 dealt with according to the requirements of the case, by means either of Fresnel's 

 beehive fixed apparatus or of a differential lens — an instrument whicli the author 

 has elsewhere described. In order to test the practicability of such an arrange- 

 ment, a mirror was constructed of small glass facets, which were arranged optically 

 on a surface of putty, and which answered the purpose as far as was possible with 

 plain pieces of glass. The author sees therefore no gi-eat difficulty in making this 

 new kind of mirror of separate facets of silvered glass of small size ; but he found 

 such difliculties in constructing one with a continuous surface that he consulted 

 his friend Professor Tait, who kindly gave him his assistance in the solution of 

 this difiicult problem by suppljang the general formula ; and he lias no doubt, now 

 that the simpler form of facet has been so successfully constructed, a differential 

 holophote will soon be made. 



Notice of the Eesearches of the late Rev. William Vernon Uarcourt, on the 

 Conditions of Transjmrenci/ in Glass, and the Connexion hetiveeti the 

 Chemical Constitution and Optical Properties of different Glasses. By 

 Professor G. G. Stokes, F.B.S. 

 The preparation and optical properties of glasses of various compositions formed 



