OCULAR SPECTRA. 107 



serve differently coloured spectra, as in the following 

 table : 



Colour of the Colour of the Spec- 



Original figures. tral figures. 



Eecl, Bluish-green. 



Orange, Blue. 



Yellow, Indigo. 



Green, Keddish-violet. 



Blue, Orange-red. 



Indigo, Orange-yellow. 



Violet, Yellow. 



White, Black. 



Black, White. 



The two last of these experiments, viz., white and black 

 figures, may be satisfactorily made by using a white 

 medallion on a dark ground, and a black profile figure. 

 The spectrum of the former will be found to be black, 

 and that of the latter white. 



These ocular spectra often show themselves without 

 any effort on our part, and even without our knowledge. 

 In a highly-painted room illuminated by the sun, those 

 parts of the furniture on which the sun does not directly 

 fall have always the opposite or accidental colour. If 

 the sun shines through a chink in a red window-curtain, 

 its light will appear green, varying, as in the above table, 

 with the colour of the curtain; and if we look at the 

 image of a candle reflected from the water in a Hue 

 finger glass, it will appear yellow. Whenever, in short, 

 the eye is affected with one prevailing colour, it sees at 

 the same time the spectral or accidental colour, just as 

 when a musical string is vibrating, the ear hears at the 

 same time its fundamental and its harmonic sounds. 



If the prevailing light is white and very strong, the 

 spectra which it produces are no longer black, but of 

 various colours in succession. If we look at the sun, for 

 example, when near the horizon, or when reflected from 



