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 that special relationship supposed by Melloni and others to subsist between the optic nerve and the oscillating periods of luminous bodies. Like a musical string, the optic nerve responds to the waves with which it is in consonance, while it refuses to be excited by others of almost infinitely greater energy, whose periods of recurrence are not in unison with its own.

10. Persistence of Rays.

At an early part of this Lecture it was affirmed that when a platinum wire was gradually raised to a state of high incandescence, new rays were constantly added, while the intensity of the old ones was increased. Thus in Dr Draper's experiments the rise of temperature that generated the orange, yellow, green and blue rays augmented the intensity of the red ones. What is true of the red is true of every other ray of the spectrum, visible and invisible. We cannot indeed see the augmentation of intensity in the region beyond the red, but we can measure it and express it numerically. With this view the following experiment was performed. A spiral of platinum wire was