Page:The wonders of optics (1869).djvu/110

 chloride of silver was blackened more quickly by the violet portion of the spectrum than by any other. In 1801, Ritter of Genoa, in repeating certain experiments made by Herschel, found that a much stronger blackening effect was produced at a point beyond the violet, and that the discoloration was produced with less intensity by the violet and still less so by the blue, the change gradually decreasing till the red ray was reached. He also found that when slightly blackened chloride of silver was exposed to the effects of the red rays, or even in the space beyond, its colour was restored to it. From these facts he drew the conclusion that in the solar spectrum there existed two kinds of rays, one at the red extremity, which favoured oxygenation; the other, at the blue end, which possessed the contrary properties. He also found that when phosphorus was placed in the invisible rays beyond the red, it gave off fumes of oxide, which were immediately extinguished when it was transferred to the other end.

On repeating the experiment with chloride of silver, Lubeck found that the tint varied according to the colour in which it was placed. Beyond or in the violet ray it became brownish red, in the blue it became bluish or bluish grey, in the yellow it remained white, or became slightly yellow and reddish in or beyond the red ray. When he used prisms of flint glass, the chloride of silver was discoloured beyond the visible limits of the spectrum.

Without being aware of Ritter's experiments, Dr. Wollaston obtained the same results by acting on chloride of silver with violet light. In continuing his researches he discovered that gum guaiacum was also influenced by the chemical rays of light.

The magnetic influence supposed to be exerted by the solar rays still remains without positive proof, although numbers of philosophers have experimented in this