JUNE 21, 1912] 
to pass before Virchow established (1855) 
the first pathological institute and as many 
again before this great master was to an- 
nounce the doctrine of cellular pathology ; 
and finally, it was thirteen years before 
Lister’s first publication concerning the 
antiseptic treatment of wounds. 
In all these activities and those which 
followed, the ideal of seeking for the truth 
no matter where it might lead—the ideal of 
pure science—was the secret of that won- 
derful progress which medicine has made 
in the last seventy-five years. : 
Now, however, it is time to return to our 
text, ‘‘In the fields of observation, chance 
favors only the mind which is prepared.”’ 
What did Pasteur mean by ‘‘chance’’? 
His meaning is very evident in his example 
of Oersted and the magnetized needle. The 
mind which is trained to observe the details 
of natural phenomena, and to reason con- 
cerning the bearing of known laws on such 
phenomena, is the ‘‘prepared mind,’’ that is 
to say, it is a class of mind which, because 
it is endowed with a peculiar faculty, best 
deseribed as scientific imagination—egrasps 
the significance of a new observation, or of 
a variation from a known sequence of 
events, and thus establishes a new law or 
invents a new practical procedure. To no 
man perhaps is this adage of Pasteur more 
applicable than to himself. It was his 
work in chemistry and his studies in erys- 
tallography that gave him the “‘prepared 
mind’’ which correctly interpreted the sig- 
nificance of the chance observation that the 
presence of a vegetable mould, the Peni- 
ciltium glaucum, in solutions of salts of the 
tartaric acids, changed an optically inac- 
tive to an optically active fluid. He 
erasped at once the true interpretation of 
this reaction. The disappearance of the 
dextro-tartarie acid, the permanence of the 
levo-tartarie acid, could be explained only 
by the assumption that the ferments of this 
particular fermentation ‘‘feed more readily 
SCIENCE 
943 
on the right than on the left molecules.’’ 
So did “‘chanece’’ direct the ‘‘prepared 
mind’’ to those fundamental observations 
which established our present-day prin- 
ciples of fermentation, and which, as the 
result of work on alcoholic, acetic, lactic 
and butyric fermentation, led to Pasteur’s 
final dictum: 
The chemical act of fermentation is essentially 
a correlative phenomenon of a vital act beginning 
and ending with it. 
It was but a short step for the mind 
thoroughly familiar with the principles of 
fermentation to embrace the opportunity 
offered by the study of the etiology of the 
infectious diseases, and so through all his 
work, as that in connection with the silk- 
worm problem, vaccination against chicken 
cholera and anthrax and the treatment of 
rabies, the ‘“prepared mind’’ of the great 
master saw and appreciated the significance 
of every observation and every opportunity 
which presented itself. 
Many other examples might be presented, 
as Semmelweis and his observations on the 
high mortality from puerperal sepsis among 
those under the care of students fresh from 
the dissecting and autopsy room and the 
low mortality among patients under other 
supervision. So also Lister and his anti- 
sepsis; and best of all, perhaps, for pur- 
poses of illustration, the sequence of Ehr- 
lich’s discoveries. We are told that in his 
student days Ehrlich was interested above 
all other things in the study of chemical 
affinities and worked incessantly with the 
new anilin dyes. Indeed the story goes 
that so engrossed was Ehrlich in his work 
that neglect of the required studies gave 
rise to some question concerning his right 
to receive his degree. The situation as 
deseribed by Christian A. Herter* was as 
follows: 
® Herter, C. A., ‘‘Imagination and Idealism in 
the Medical Sciences,’’? Jour. Am. Med. Asso., 
LIV., p. 423, 1910. 
