58 
R. KOCH. 
bacilli are at one time harmless, at another time disease-producing parasites* 
These conclusions are so indisputable that probably nobody questions them, and 
that in science the bacilli of inflammation of the spleen are considered the 
cause as well of the usual typical disease as appearing in our domestic animals, 
as also of the clinically varying type which appears in man. 
The course just sketched, which those who strove to prove the parasitic 
nature of inflammation of the spleen have taken with success, and the con¬ 
clusions necessarily obtained from the results, I have placed at the foundation of 
my investigation of the aetiology of tuberculosis. These had to employ them¬ 
selves in the first place with the proof of disease-producing organisms, then with 
their isolation, then with their inoculation. I now go over to the description of 
these single divisions of the investigation. 
I .—Proof of the 'presence of disease-producing organisms in the organs changed by 
tuberculosis and in the separation of the latter. 
Disease-producing organisms, which have the size of inflammation-of-the- 
spleen bacilli and like these, appear in the blood in large numbers, or those which, 
like the ‘ ‘ nekurrensperochaeten ” attract the eye, offer no especial difficulties in 
investigation, and the proof of such micro-organisms can be obtained by ordinary 
optical helps. The case is different, however, when it is necessary to prove the 
existence of minute bacteria present in the tissues in only small numbers, especi¬ 
ally when cells are heaped up or broken down in the spots concerned, which is 
almost always the case. Then it is necessary to use the more delicate technical 
helps of microscopy, as especial methods of preparation and differential coloring, 
and to pursue the investigations with the best optical apparatus, oil-immersion 
systems and Abbe’s illuminating apparatus. 
Also in regard to tuberculosis it was to be expected that to show that special 
disease-producing organisms do actually exist, might offer special difficulties, as 
they had already been much sought for, and nothing found that could give con¬ 
fidence in their existence. I began investigations with material in which infec¬ 
tious matter might be expected with certainty, for instance, in freshly developed 
and still grey tubercles from the lungs of animals which had been killed three or 
four weeks after inoculation. From these lungs, hardened in alcohol, sections 
were made and examined according to the most approved methods for proving 
the existence of bacteria. Grey tubercles were also crushed, spread out on glass 
covers, dried, and then examined with reference to the existence of micro-organ¬ 
isms. All attempts to find bacteria or other micro-organisms in these prepared 
specimens proved unsuccessful. In former experiments it had been attempted to 
color the bacteria as strongly and as differently from the surrounding tissue as 
possible, and in such attempts it had been proved that in certain cases the addi¬ 
tion of alkaliefe to the color-solutions offered essential advantages; therefore this 
treatment was adopted. Of the usual analine color the methyline blue will bear 
the greatest addition of alkali, on which account this coloring material was 
chosen, and just so much potash-lye was added to a watery solution of the same 
as to form no precipitate, and that the liquid remained clear. For the prepara¬ 
tion of this mixture 1 ccm. of a concentrated alcoholic methyline-blue solution 
and 200 ccm. of distilled water were mixed, well shaken and during repeated 
shaking 0.2 ccm. of 10 proc. potash-lye was added. When glass-covered prepara- 
