JOURNAL

OF THE

ROYAL MICROSCOPICAL SOCIETY.

SEPTEMBER, 1878.

I.-On the Measurement of the Diameter of the Flagella of Bacterium termo: a Contribution to the Question of the "Ultimate Limit of Vision" with our present Lenses. By Rev. W. H. DALLINGER.

(Read before the ROYAL MICROSCOPICAL SOCIETY, June 5, 1878.)

PLATES VIII. AND IX.

Ir will be remembered by this Society that two years ago, having in conjunction with Dr. J. J. Drysdale completed as far as we then purposed a series of observations on the life-histories of a group of monads, we determined to use the experience thus gained, and if possible to study in a similar way the Bacteria. We commenced on B. termo; and our first object was to make out clearly its normal form, and to discover if possible the agency by which movement was effected.

In the case of the large form known as Spirillum volutans,

7.-Photograph of Bacillus subtilis, showing flagella. By Dr. Koch × 500. 8.-Ditto, ditto (or as Dr. Koch thinks a variety, B. amylobacter), also photographed, and faintly showing flagella × 700 diam.

FIG. 9.-Bacillus subtilis × 4000 diam.

10.-B. termo.

11.-A camera lucida outline of the lower part of B. termo, magnified 2000 diam. c is the pencil line intended to cover the image of a part of the flagellum; and a represents by dots the part of the image with which the line can be compared.

FIG. 12 is the above drawing placed on the stage of the microscope, and magnified 5 diam. In this way the ratio between the diameter of the body of the Bacterium and the diameter of the flagellum can be compared by means of the wires of a screw micrometer. The ratio here shown being as 10 to 1.

VOL. I.

Cohn had demonstrated, what Ehrenberg had suspected, and we had readily confirmed, that its movements were produced and controlled by a pair of fine flagella, one at each end of its spiral body. This was extremely suggestive: while we had further the analogy of the minuter monads, many of them being only four or five times larger than the larger forms of B. termo-these also being endowed with one or more flagella, suggesting the probability that all these forms depended for movement on similar motile filaments.

We therefore determined to endeavour to discover whether, by care and delicate manipulation, flagella could be demonstrated on B. termo itself.

Fortunately at this time we were furnished with a "new formula" lens, which possessed precisely the qualities needed. Carrying out our method of admitting nothing, in our conjoint work, which both had not seen, I commenced the search; and after many hours' effort, and the use of a great variety of delicate appliances, I succeeded in clearly demonstrating a pair of flagella, one at each end of the body of B. termo, without the shadow of a question. Dr. Drysdale now, independently, commenced the search, simply employing the same methods, and ultimately demonstrated it as completely as could be desired. We were then able to study it together, seeing the delicate fibre as distinctly as the body of the B. termo itself, and I made careful drawings from this, which Dr. Drysdale accepted and confirmed.*

Most of the forms at present grouped as Bacteria vary, very considerably, in size. Some, for example, that may be designated B. termo will be as large again as others; and this is the case throughout, only that the variation will be still greater in those forms which, instead of dividing into two parts, divide into several. There is for instance very great variety in the length of S. volutans. But the average length of B. termo, resulting from a hundred measurements made from this form as taken from six different infusions, was the robooth of an inch. The average of one hundred measurements of S. volutans (taken only from one infusion) was the Tooth of an inch, but these were large. In Fig. 2, Plate VIII., I give a drawing of a recent specimen of S. volutans magnified 2000 diameters; and at Fig. 1 a B. termo is placed beside it, as it was seen by Dr. Drysdale and myself, magnified 4000 diameters.

Since this account of the discovery of flagella on B. termo was published, several valuable monographs have appeared on the subject of Bacteria. From the point of view I am now specially considering one of the most interesting and valuable is by Dr. Eug. Warming. In it he points out that the flagella are common to the

'Monthly Microscopical Journal,' vol. xiv. p. 105.

↑ Om nogle ved Danmarks Kyster levende Bakterier.' Kjöbenhavn, 1876.

group, and that in some cases they have two flagella at one end, a fact which I have frequently confirmed. But there is an immense difference between all the other forms and B. termo, in size. It may be helpful to realize what this difference is: and I have given copies of drawings from nature of all that Warming and others give as possessing motile filaments, which I have been able to confirm. They are each magnified as indicated, either to the same extent as B. termo, or to two-thirds or a half that extent. Bacillus ulna and Bact. lineola (Figs. 5 and 6) I have not hitherto seen as included in the list of those possessing flagella; but they are given as drawn by camera lucida from nature during my investigations for this paper. Their flagella are certainly not more difficult to demonstrate than the flagellum of the uniflagellate monad whose history was studied and described by Dr. Drysdale and myself in January 1874.* A comparison then of Figs. 2, 3, 4, 5, and 6 with Fig. 1 will indicate as accurately as may be the relative sizes of the varieties now known to be possessed of flagella, when compared with B. termo.

But a still more instructive instance is before me. Dr. Koch, in a paper recently published † has given illustrations taken by micro-photography, of what he considers to be Bacillus subtilis (or a variety), in which the flagella are distinctly visible in the print. I send these photographs, which can doubtless be reproduced by lithography. In Fig. 7, Plate IX., the magnifying power is 500 diameters, and the flagella are distinctly visible. In Fig. 8 the magnification is 700 diameters, and the form shown at a displays what are doubtless its flagella, although they are but feebly and faintly manifest. These photographs were, however, not taken while the organisms were living, nor in their natural fluid. They were, indeed, specially prepared, by drying, staining, and mounting in glycerine; and then photographed. This must to a certain extent superinduce change of condition, and even alteration of form. This is manifest in the relative thickness of the flagella and the bodies of the forms photographed, which is certainly not in all cases as it presents itself in the living organism; this is specially seen in Fig. 7. But the wonder is that they are so successfully, and even beautifully done. In Fig. 9 I give a drawing from nature of B. subtilis magnified 4000 diameters, to show at once, the relation of this form as a "species" to B. termo, and to show the condition in which those must have been which Dr. Koch photographed; namely, the separated segments, such as a, b, which become provided, like B. termo, with flagella, as they divide.

*M. M. J.,' vol. xi. p. 69.

† 'Verfahren zur Untersuchung, zum conserviren und Photographiren der Bacterien,' Von Dr. Koch. Beitrage zur Biologie der Pflanzen. Edited by Dr. F. Cohn. Third part, 1877.