572 THE POPULAR SCIENCE MONTHLY. 



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 par- 

 ticularly to the use of vaccine as a preventive of small-pox. 



The works of Darwin being now familiar to physicians, it is prob- 

 able 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. 



+•» 



MODERN OPTICS AND PAINTING. 



Br 0. N. ROOD, 



PROFESSOR OF PHYSIOS DT COLUMBIA COLLEGE. 

 II. 



LET 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 atten- 

 tion 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. Mtiller, 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 



