_Two bright lights may be made to produce darkness._--If two pencils of light radiate from two spots very close to each other in such a manner that they cross each other at a given point, any object placed at that line of interference will be illuminated with the sum of the two luminous pencils. If we suppose those rays to move in waves, and the elevation of the wave to represent the maximum of luminous effect, then the two waves meeting, when they are both at the height of their undulation, will necessarily produce a spot of greater intensity. If now we so arrange the points of radiation, that the systems of luminous waves proceed irregularly, and that one arrives at the screen half an undulation before the other, the one in elevation falling into the depression of the other, a mutual annihilation is the consequence. This fact, paradoxical as it may appear, was broadly stated by Grimaldi, in the description of his experiments on the inflection of light, and has been observed by many others. The vibratory hypothesis, seizing upon the analogy presented by two systems of waves in water, explains this plausibly, and many similar phenomena of what is called the _interference of light_; but still upon examination it does not appear that the explanation is quite free from objection.[98] Another theory, not altogether new to us, it being indicated in Mayer’s hypothesis of three primary colours (1775), and to be found as a problem in some of the Encyclopædias of the last century, has been put forth, in a very original manner, by that master-mind of intellectual Germany, Goethe; and from the very comprehensive views which this poet-philosopher has taken of both animal and vegetable physiology (views which have been adopted by some of the first naturalists of Europe), we are bound to receive his theory of colours with every respect and attention. Goethe regards colour as the “thinning” of light; for example, by obstructing a portion of white light, yellow is produced; by reducing it still farther, red is supposed to result; and by yet farther retarding the free passage of the beam, we procure a blue colour, which is the next remove from blackness, or the absence of light. There is truth in this; it bears about it a simplicity which will satisfy many minds; by it many of the phenomena of colour may be explained: but it is insufficient for any interpretation of several of those laws to which the other theories do give us some insight. Newton may have allowed himself to be misled by the analogy presented between the seven rays of the spectrum and the notes in an octave. The mystic number, seven, may have clung like a fibre of the web of superstition to the cloak of the great philosopher; but the attack made by Goethe upon the Newtonian philosophy betrays the melancholy fact of his being diseased with the lamentable weakness of too many exalted minds--an overweening self-esteem.