172 G. Tait, Esq., on producing White or Neutral Light y 



a portion of these rocks, and explain the reasons of it, to obtain from 

 this gentleman a ready and courteous compliance, though involving 

 some change of his plans. I only hope, that he will not put over 

 these rocks a sign-board, to inform the public, that they are " Glacial 

 Hocksy''^ — ^which is the term he applies to them, in his correspondence 

 with me on the subject. 



On producing White or Neutral Light, hy means of ordinary 

 Artificial Light, By George Tait, Esq., Advocate, Vice- 

 President of the Royal Scottish Society of Arts. Commu- 

 nicated by the Royal Scottish Society of Arts.* 



It is generally known that ordinary artificial light, such 

 as that obtained by the combustion of wax, tallow, oil, or car- 

 buretted hydrogen gas from coal, produces a tinge of colour 

 very different from that produced by the white light of the 

 sun. This is found, upon examination, to be an orange tinge, 

 so exceedingly strong as to reduce it to orange colour, slightly 

 modified by blue, as will be seen presently in detail. The 

 two lights may be compared, in a general way, by observing 

 the colours of different objects enlightened by them sepa- 

 rately at the same time ; for example, throwing contiguous 

 shadows of an opaque body from each of them upon a white 

 surface, and comparing the colours of the shadows, the shadow 

 from each light shewing the colour of the other light which 

 alone falls upon it ; the white, however, if not in large pro- 

 portion, assuming a bluish appearance from contrast with the 

 orange all around, of which blue is the complementary colour. 

 This orange tinge necessarily affects very much the appear- 

 ance of the colours of the objects exposed to artificial light. 

 And it would be of importance for some of the useful arts, in 

 which it is necessary to distinguish colours, for the depart- 

 ment of painting in the fine arts, whether for executing or for 

 viewing pictures, and for other purposes generally, if light 

 free from tinge and of sufficient strength could be produced 

 by means adapted for ordinary use. And, accordingly, this 



* Re»d and exhibited before the Society, 23d November 1846. 



