Nothing can be more superb than the green of the Atlantic waves when the circumstances are favourable to the exhibition of the colour.  As long as a wave remains unbroken no colour appears, but when the foam just doubles over the crest like an Alpine snow-cornice, under the cornice we often see a display of the most exquisite green.  It is metallic in its brilliancy.  The foam is first illuminated, and it scatters the light in all directions; the light which passes through the higher portion of the wave alone reaches the eye, and gives to that portion its matchless colour.  The folding of the wave, producing, as it does, a series of longitudinal protuberances and furrows which act like cylindrical lenses, introduces variations in the intensity of the light, and materially enhances its beauty.

We are now prepared for the further consideration of a point already adverted to, and regarding which error long found currency.  You will find it stated in many books that blue light and yellow light mixed together, produce green.  But blue and yellow have been just proved to be complementary colours, producing white by their mixture.  The mixture of blue and yellow pigments undoubtedly produces green, but the mixture of pigments is a totally different thing from the mixture of lights.

Helmholtz has revealed the cause of the green produced by a mixture of blue and yellow pigments.  No natural colour is pure.  A blue liquid, or a blue powder, permits not only the blue to pass through it, but a portion of the adjacent green.  A yellow powder is transparent not only to the yellow light, but also in part to the adjacent green.  Now, when blue and yellow are mixed together, the blue cuts off the yellow, the orange, and the red; the yellow, on the other hand, cuts off the violet, the indigo, and the blue.  Green is the only colour to which both are transparent, and the consequence is that, when white light falls upon a mixture of yellow and blue powders, the green alone is sent back to the eye.  You have already seen that the fine blue ammonia-sulphate of copper transmits a large portion of green, while cutting off all the less refrangible light.  A yellow solution of picric acid also allows the green to pass, but quenches all the more refrangible light.  What must occur when we send a beam through both liquids?  The experimental answer to this question is now before you:  the green band of the spectrum alone remains upon the screen.

The impurity of natural colours is strikingly illustrated by an observation recently communicated to me by Mr. Woodbury.  On looking through a blue glass at green leaves in sunshine, he saw the superficially reflected light blue.  The light, on the contrary, which came from the body of the leaves was crimson.  On examination, I found that the glass employed in this observation transmitted both ends of the spectrum, the red as well as the blue, and that it quenched the middle.  This furnished an easy explanation of