by means of ordinary Artificial Light, 185 



stained or painted of a tint of blue, of which he gives a speci- 

 men, in order to absorb the orange, and to throw down the 

 blue rays to qualify the orange light below. But upon try- 

 ing it over an argand flame, with the tint prescribed, and 

 also with a darker tint, I found that the orange tint below 

 was not materially subdued. He also proposes the absorp- 

 tion of the excess of orange by transmission through a blue 

 medium. One of the modes proposed, is to have the chimney 

 of an argand lamp- stand of a *' very pale blue colour," and 

 to throw down the light transmitted through it by a bright 

 conical metallic reflector. But this description of the tint of 

 blue is very indefinite, and it does not appear to indicate 

 nearly the depth of tint necessary for neutralization, which 

 absorbs so large a portion of the light as before mentioned. 

 Another mode proposed, is to transmit the light of an argand 

 flame, or a flat flame, by a similar reflector, through a disc of 

 glass under the flame, stained blue of such a depth of tint, 

 that a piece of white paper seen through it in daylight, may 

 appear " nearly of the colour of the blue sky,"" or for wood 

 engraving, and some other such purposes, to transmit the 

 light, having a common white reflector over the flame, through 

 a large globular glass bottle, containing water, coloured ac- 

 cording to the same standard. But the intensity of the blue 

 of the sky varies through every tint, from the darkest to the 

 palest, according to the latitude of the place, the state of the 

 atmosphere, or the part of the sky observed, the blue towards 

 the zenith being, in general, many times darker than that to- 

 wards the horizon. It will be observed also, that, by those 

 modes of absorption, more or less of the artificial light is al- 

 lowed to escape unmodified. Whereas, by the method which 

 I have suggested, this is prevented ; and, by adjusting the 

 colour in unison with white natural light, a standard is at 

 once obtained, which is precise, correct, and invariable, and 

 which secures the production of pure white light fit for any 

 use in the arts. 



