430 Dr. E. L. Nichols's new Explanation of 



those striking changes of hue due to variations of the intensity 

 of illumination. Of the simple colours, as the brightness of 

 the ray increases, red and green change to yellow, blue becomes 

 white. Besides these remarkable changes, familiar only to 

 the spectroscopist, there are in the outdoor world a host of 

 striking effects. Passing cloud-shadows on the water at noon- 

 day reveal to us that what under shadow is indigo-blue be- 

 comes instantly, in the direct sunlight, a subtile silver-grey. 

 The same cloud-shadows transform the sun-lit yellow-green of 

 a neighbouring hillside into blue-green. All these effects are 

 the necessary result of that peculiarity in the action of our 

 nerve-termini. Of more direct interest at present is the in- 

 fluence upon white daylight. Such light at ordinary intensi- 

 ties affects the three sets of nerve-termini equally ; the re- 

 sultant impression is whiteness. Now daylight is simply the 

 light of the sun weakened by manifold diffuse reflexions. 

 The direct rays of the sun as we let them fall upon any colour- 

 less object appear also a white light ; but on attempting at 

 noon of a clear day to gaze into the sun's face, the impression 

 is of blinding yellow. It is not that the direct rays differ in 

 composition from diffuse daylight, but that the " violet "- 

 nerves are ill adapted to transmit the action of such strong 

 light. They shrink from their share of the work, bewildered 

 and overcome, while the " green " and " red "-nerves are 

 stimulated by the glare to extraordinary activity. 



In nature the variety in the intensity of light is almost in- 

 finite. The moon with -5- o ^o oo? perhaps, of the sun's illumi- 

 nating-power seems bright ; and she is in reality far brighter 

 than the open sky. At noonday even she appears like a white 

 cloud, around which the blue atmosphere often looks almost 

 black by contrast. In passing from the intensity of the 

 moon's rays to those reaching us from a corresponding bit of 

 the open sky, we may perhaps take a step as great as that 

 between the brightness of sun and moon. Already in the 

 case of the moon's light the yellow impression given by direct 

 sunlight has been supplanted by a blue one*. The contrast 

 between the colour-impressions due to sunlight and to moon- 

 light is far greater than we, in the lack of a means of direct 



* This effect is of course frequently vitiated by the fact that the moon's 

 rays on their way to us have to penetrate dense strata of aqueous vapour. 

 Such vapour, as shown by Forbes (" On the Colour of Steam," Edinburgh 

 Trans. 1843), absorbs all the more refrangible rays ; and moonlight then 

 appears golden, or even reddish. That blue is nevertheless the hue com- 

 monly associated in our minds with moonlight, the Venetian " moon- 

 light " photographs (so called) bear witness. They owe their effect to 

 the blue wash which converts a common daylight picture into the desired 

 moonlight view. 



