20 Traxsactions of the American Institute. 



limitations to the information whicli sic^ht can give which are very 

 soon reached. In the examination of structure, in the study of form, 

 iu the observation of the movements and changes continually going 

 on in organic things, we presently arrive at a point at which further 

 progress is arrested by the imperfection of our powers. More than 

 this; while thus vainly seeking to know something of the minute 

 organization of bodies large enough to be seen and examined in mass, 

 without difficulty, we make the new discovery that there exist many 

 objects of a high order of interest M'hich, in their fully developed 

 proportions never attain a magnitude sufficient to betray even their 

 existence to ordinary vision, and of which, without artificial helps, 

 w.e could never know anything at all. And yet, it can easily be 

 made to appear that on the knowledge which we may be able to 

 attain of this class of objects must depend to a great degree the 

 progress which is to be made in future toward the amelioration of the 

 condition of the human race. Sanitary science, so far as there is such 

 a ^science, rests at present upon principles to a great extent empirical. 

 It.is becoming every day more and more clear that the causes of all 

 zymotic diseases are to be sought in excessively minute and widely 

 scattered organisms, which to ordinary observation are totally imper- 

 ceptible. This, it may be further remarked, is just as true of the 

 diseases of plants as those of animals. The potato rot, the cotton 

 rust the smut of wheat, and the wasting of the vine, are just as 

 certainly the product of microscopic fungi as the rinderpest, the 

 epidemic among silk-worms, or Xhe cholera among men. 



To the study of objects of this kind, the microscope is absolutely 

 indispensable. It is to the rai(;roscope, indeed, that we owe the 

 knowledge that any such things exist. But supposing us to have 

 been otherwise possessed of so much knowledge, it is still true that 

 without this instrument we could know nothing beyond. We could 

 know nothing of their modes of development, the conditions favor- 

 able to their multiplication, the manner of their diffusion, or the 

 means best adapted to prevent their appearance or check their 

 growth. This signal example of the use of the microscope I men- 

 tion first, because of the magnitude of its importance, and because 

 of tlie recency of its recognition. The application of the instrument 

 to the study of disease generally, and especially to morbid anatomy, 

 has long been familiar. So large are the services which it has thus 

 rendered to the healing art, that, in the words of an eminent 

 wi-iter on this subject, " The smallest portion of a diseased structure 



