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68 R. KOCH. 
that it be used exactly according to the given directions. It would be of special 
interest to be able to give the bacilli a brown or yellow coloring, because only 
under this fcondition would it be possible to get usable photographs of the tubercu¬ 
lous bacilli. Of late I have, to be sure, succeeded, with the help of a previously 
given treatment of the preparations, with a very weak solution of kali (yV p. M.) 
to color the tuberculous bacilli a quite intense brown, nevertheless the preparations 
do not meet the demands required for photography. It is to be hoped that this 
difficulty will be overcome. But for the present I have been obliged to do with¬ 
out photographs, however much I have wished by means of photographs to render 
possible a reliable comparison between the form and size of tuberculous bacilli 
and other similar ones. 
As another hindrance in the coloring of the tuberculous bacilli the temporary 
character of the coloring must be mentioned. After a shorter or longer time in the 
preparations enclosed with Canada balsam, the color of the bacilli begins to lose 
its intensity, very gradually it becomes less noticeable and finally vanishes 
completely. The preparations colored with methyl-violet and gentian violet 
pale most quickly, for in some cases the color of such bacilli vanished in two days. 
The preparations colored with fuchsine keep much longer, as do those colored 
with alkaline methyline blue solution. Why it is that the color is so fleeting, 
while the same color in the dyeing of other bacteria have proved themselves un¬ 
changeable for years, I am not able to say. But from the circumstance, that in a 
great number of prepared specimens single ones have been found which have 
preserved the color completely unchanged for almost a year, I must conclude, that 
some sort of conditions are present and may be found, which will make possible 
the retaining of the color. 
Moreover the preparations which are so pale have not become entirely useless, 
as with little trouble they can be colored again. The Canada balsam must be 
liquified by heat, the specimen taken off carefully with a pinsel and put into oil of 
turpentine. After 24 hours it is laid in absolute alcohol and after another 24 hours 
into the color solution to go through the whole coloring process again. The tu¬ 
berculous bacilli tak6 the blue color just as intensely as at first, but their surround¬ 
ings, on the contrary, appear less beautifully and clearly colored than before. 
A reliable explanation of the difference between tuberculous bacilli and other 
bacteria in their action in regard to coloring matters appears to me impossible for 
the present, on account of the insufficient knowledge of the more delicate structure 
of the chemical constitution. On many grounds it seems likely that the tuber¬ 
culous bacilli are surrounded by a coating, which acts differently toward coloring 
matter than the contents, as we already know to be the case with other bacilli. 
The bacilli dyed with methyline blue appeared thinner than those dyed with 
methyl violet or fuchsine. One sees in the groups in which the bacilli lie closely 
pressed together, that the methyl violet colored bacilli move, and the bacilli dyed 
with methyline blue and appear thin, are separated from each other by plainly 
manifest spaces. Further, the coloring of the bacilli intensely dyed with methyl 
violet in growing pale does not vanish uniformly, but an outer layer grows pale 
first, so that of the thick bacillus a thinner still intensely thread remains, which 
possesses about the thickness of the bacillus colored with methyline blue. Finally 
the firm cleaving together of the bacilli in the groups also speaks for the presence 
