130 Moonlight and Colour. 



green or the bright blue one, so far as change in the colour 

 of the partially lit portion is concerned. 



Another experiment may be made by taking three books, 

 bright red, blue, and green, into a dark cupboard, and letting 

 in little light by slow degrees. The red will be seen first, 

 the green next, and the blue last. Yellow would have been 

 visible before the red. 



As no object possesses an inherent property called colour, 

 but simply a power of acting in a definite manner upon light, 

 absorbing or reflecting more or less of the seven prismatic 

 colours, or so-called components of white light, it follows that 

 what we call yellow, red, or blue objects cannot be seen as 

 such, unless enough of the right kind of light falls upon them, 

 and the whitest body looks dark in proportion as white light 

 is prevented from reaching it, and, when only just enough 

 white light is admitted to render a sheet of white paper visible, 

 it has a dark grey aspect, such as would be obtained by grind- 

 ing up enough charcoal with a bit of chalk to nearly destroy 

 the whiteness of the latter. 



These facts are necessary as a preliminary towards under- 

 standing the effects of moonlight. The amazing luminosity of 

 the sun has been already spoken of, and Woollaston estimated 

 it as being 801,072 times brighter than the full moon;* 

 but taking it at half this, the difference will be immense. 

 When, therefore, moonlight is substituted for sunlight, an 

 eriormous change must be made in the intensity of all colours, 

 and to such an extent does this take place that many objects 

 look dark, or resemble shadows, in moonlight; whereas in 

 sunlight they may appear bright and of a strongly expressed 

 tint. When light is reflected from the surface of a coloured 

 object, more or less luminosity is lost, according to the kind of 

 colour and the nature of the surface. If the intensity of the 

 light is sufficiently diminished, the colour may vanish and be 

 replaced by a nondescript hue, while the form remains visible. 

 Now, ordinary moonlight is not intense enough, even to indicate 

 what many coloured objects would look like in sunlight, 

 although bright primary or bright secondary colours can" be 

 approximately made out. Light loses intensity when it passes 

 through transparent bodies, such as coloured glass; and a piece 

 of blue or red glass, which distinctly gives its own tincture to 

 solar rays, will, under some circumstances, appear not to tinge 

 the feebler rays of the moon. 



If a sheet of white paper is held so that it receives sun- 

 light that has passed through coloured glass, a strong tint is 



* Michelo and Kulcr estimated it on theoretical grounds at 450,000 and 

 374,000, and Bouguer by measurement of shadows of the light of wax tapers at 

 only 300,000. Sec Cosmoi (Bohn), vol. iii., p. 127.] 



