for the purposes of Illumination in Lighthouses. 65 



The system of illumination by lenses, may therefore be con- 

 sidered as absolutely necessary to the proper construction of co- 

 loured distinguishing lights, in so far as this system will alone 

 enable us to dispense with the use of red light, the very colour 

 which the atmosphere itself can produce. But there is another 

 most important consideration, which renders the lenticular system 

 peculiarly adapted to coloured lights. While a large sheet of co- 

 loured glass is necessary for colouring the column reflected from 

 a parabolic reflector, we may accomplish the same purpose in len- 

 ses, by means of a small plate of coloured glass three or four inches 

 square, placed as close as can be conveniently done, to the illu- 

 minating flame, which will colour the whole column of light as 

 effectually as if it had been of the same diameter as the lens. 

 This facility of applying coloured media, will enable us to avail 

 ourselves of natural and artificial substances, which could not 

 possibly be procured in large plates *. Yellow orpiment, for ex- 

 ample, or sulphate of copper, and various other substances, 

 might be placed, in thin pieces, between two plates of glass, so 

 as to form a square-coloured plate, sufficiently large to receive 



* This advantage is strikingly pointed out by the following fact stated by Mr STE- 

 VENSON : " After having corresponded with all parts of the kingdom in endeavouring 

 to procure red glass of the finest quality, by having it coloured in the furnace, it was 

 mortifying to find, that its manufacture was wholly impracticable, excepting' in the pro- 

 duction of small pieces not more than three or Jour square inches, similar to those in 

 the compartments of cathedral windows, which, in the process of shading a reflector, 

 must have induced a number of minute divisions, and necessarily obstructed much 

 of the light. The writer at length resolved on confining his attention to plates of 

 crown-glass stained by repeated application of the litharge of gold, laid on after the 

 manner of gum or paint, which was afterwards subjected to a strong heat in a muffled 

 furnace of a peculiar construction, forming altogether a very nice and difficult pro- 

 cess. ***** Although the effect produced in this way cannot be so perfect as 

 if the glass were uniformly coloured in the pot, yet, when applied to the purposes of 

 a distinguishing light, its effects are highly characteristic and beautiful." Account 

 of the Bell Rock Lighthouse, p. 392. 



VOL. XI. PART I. I 



