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572 violent, but the return is evident. All means of resisting it, which would have sufficed fifty years since, have become less efficacious.

To sum all in general terms: heredity and selection must produce an alternation of intensity and relief in diseases. That variation must be more marked, when the disease in which it takes place is more fatal, and especially when it attacks youth. Curative or preventive means, which are sufficient in periods of light visitation, lose a portion of their efficacy at the aggravated periods. And this rule applies particularly to the use of vaccine as a preventive of small-pox.

The works of Darwin being now familiar to physicians, it is probable that many among them have considered the effect of the law of selection upon the variation of intensity in maladies. I doubt, how-ever, whether they have given attention to the consequences relative to vaccination. It is this which has led me to bring within the range of medical investigation an application (perhaps novel) of the ideas of the celebrated English naturalist.



ET us now pass to the examination of a theory which was proposed in 1807 by the now justly-celebrated Thomas Young who seems to have been gifted with a scientific insight much too keen for the age in which he lived. His views being opposed to the common notions of the day, commanded but little attention, and it was reserved for Helmholtz, almost half a century later, to call attention to this nearly-forgotten theory, and to show that it accounted for all the ascertained facts in a most satisfactory manner. In this work he has been ably seconded by Maxwell, and more lately by the German physicist J. J. Müller, who with improved apparatus care-fully repeated Helmholtz's original experiments, and corrected them in some minor details.

According to our new theory, then, there are in the retina of the eye, where the pictures of external objects fall, three sets of nerves, adapted for the production of three separate, distinct sensations, which we call red, green, and violet. When, owing to any cause whatever, one of these sets of nerves is excited into action, the result is the corresponding sensation; if, for example, we act upon the last set by electricity, pressure, or by the luminous waves, the result will be the sensation of seeing violet light, even though not a ray of light of any