by iman$ of otdingxx^ Artificial Light, 183 



vary from about one-thirtieth to about one-fifth part, accord- 

 ing to the quality of the glass, from pijre colourless plate- 

 glass or white flint-glass, downwards.* But if the angle of 

 incidence be reduced to 60°, there is a perceptibly greater 

 loss of light, which increases rapidly as the angle is farther 

 reduced ;t and, therefore, such obliquity ought to be avoided. 

 It consists also, in the case of painted glass, of the light lost 

 by defect of transparency, which, of course, varies accord- 

 ing to the execution. But it consists chiefly pf the great ex- 

 cess of orange in the light not stopped by the two preceding 

 causes, which is absorbed by the blue of the medium as be- 

 fore noticed. The total loss in any case is easily ascertained 

 by means of shadows in the usual manner. But it appears 

 that by using a bright flame and glass of good quality, 

 " coloured" or well painted, the loss should not exceed five- 

 sixths, or at most six-sevenths. It may be less. 



The loss of light, of course, causes a corresponding increase 

 of expense. But it appears that the objection on this ground 

 is not so great as might be anticipated. Dr Fyfe states the 

 following among the results of observations made by him 

 with regard to the expense of a variety of materials for pro- 

 ducing light, taking as the unit the expense of good coal-gas, 

 yielding 12 per cent, of matter condensible by chlorine : — Gas 

 argand, 1*00; gas swallow-tail, 1-40 ; solar oil in solar lamp ; 

 2'00 ; solar oil in common argand lamp, 3-98 ; whale oil in do., 

 5*00 ; sperm in do., 800 ; moulded candles (short sixes), with 

 two wicks, 1270, with one wick, 1350 ; wax candles, 2590; 

 Spermaceti and composition candles, 29*20. J So, that, sup- 

 posing the expense of white light to be six times that of ordi- 

 nary artificial light, it may be produced by a gas argand lamp 

 for little more than the expense of the same quantity of orange 

 light from common whale oil in an argand lamp ; for about half 

 the expense of tallow candles ; and for less than a fourth part 

 of the expense of wax candles ; and if gas cannot be obtained, 



* See Table by Sir William Ilcrschel, in Rees' Cyclopaedia, " Light." 

 t Sec Table by JI. Bouguor, ibid. 



X Transactions of the Koyal Scottish Society of Arts, vol. ii., pp. 133-147 \ 

 Edinburgh New Philosophical Journal, vol. xxxii., pp. 221-235. 



