castor-oil, an immersion-fluid can be made, which is similar in its refracting power to oil of cedar, or which differs from it to a certain degree.

In consequence of the removal of the troublesome cover-glass correction (attained by different lengths of the draw-tubes) which allows, in a delicate manner, a compensation for the influence upon the aberration of the varying distance of the picture, the objectives for homogeneous immersion are always adjusted for a definite length of the draw-tube. On this account it is to be noted that lengthening the draw-tube beyond this normal length acts in the way of a spherical over-correction, shortening in the way of an under-correction.

In using an immersion-lens, one places a drop of oil on the cover-glass, screws down the draw-tube with the coarse adjustment, or, if this is lacking, brings it down with a rotary motion by the hand, so far that the front lens of the objective touches the oil and the picture begins to be visible. Then the fine adjustment with micrometer-screw is used. Some place a drop of oil on the front lens of the objective. Others put a drop not only upon the objective, but also upon the cover-glass.

After use, the oil is carefully removed from the lens with a fine linen cloth, less satisfactorily with blotting-paper, and the system is returned to its case. If the cover-glass preparation is to be preserved, then the oil is soaked up with filter-paper, and what remains is finally removed by chloroform or benzine.

According to histological tradition, which influenced the earlier use of the microscope, the magnifying power should be increased rather through the use of a more powerful lens than a more powerful

ocular. But, according to a purely physical principle, the strength of an ocular, which an objective will allow with advantage, depends upon the angle of aperture of the latter. The larger the angle of aperture, so much the stronger, other things being equal, can the ocular be. Our best homogeneous systems correspond to this requirement, namely, that one can use the same objective and at the same time employ the most powerful oculars.

In this way, bacteria without any special preparation may be observed between the slide and coverglass. One cause of uncertainty is here noticed, i. e., almost all bacteria are in motion. This in part seems to be spontaneous motion; in part a simple Brownian molecular movement, such as occurs in all fine particles suspended in a fluid. In proportion to the minuteness of the objects, these movements render exact observation difficult. For this reason one should early eliminate this, and fix the forms of micro-organisms by narcotizing them.* For this purpose, a particle of spirituous or dilute alcoholic tincture of opium can be added with the point of a needle to a drop of water.

Moreover, von Recklinghausen + has shown that small, round, granular tissue-detritus, which is so easily confounded with bacteria, can be sharply differentiated by the fact that bacteria are pre-eminently homogeneous granules, and are totally unaffected by the action of acetic acid, glycerine, or even caustic soda.

Baumgarten succeeded in making the tubercle

* Perty, "Zur Kenntniss kleinster Lebensformen," 1852, S. 13. +"Verhandlungen der Physikal-Medizin. Gesellschaft in Würzburg," N. F., II. Bd., Heft 4, 1872. "Sitzungsberichte," S. XII. "Centralblatt f. d. med. Wissenschaft," 1882, No. 15.

bacilli visible by their resistance to a diluted solution of caustic soda, though he could not recognize them by staining according to the methods in use at that time.

We possess now more convenient means, both for fixation and differentiation. But it would be a serious error not to examine bacteria unfixed and unstained. It is, on the contrary, necessary throughout that the bacteria should be observed under the most natural conditions possible in order to study their motion, to follow the formation of spores and their germination, and to control the forms according to other treatment.

For this purpose we do not use the previously described form of investigation, but employ the moist chamber. For this the hollow slides A and B (Fig. 6) serve. A small drop (c) of the bacteria-containing

FIG. 6.

a

A

B

fluid is placed upon a cover-glass (b). The cover-glass is quickly reversed, and, with the drop now hanging underneath (c, B), is laid over the hollow (a) in the slide, and its edges are surrounded by vaseline, wax, paraffine, or balsam, in order to prevent evaporation of the fluid. Another and a better form is represented in Fig. 7. Upon a slide, A or B, a glass plate (b)

is cemented, which has a central circular opening. A chamber is formed by laying a cover-glass (a) over

the opening. This room, in place of the almost halfcircular cavity in Fig. 6, is bounded by parallel walls. The drop is prepared in a similar manner, and in the same way hangs within the chamber. These chambers can be improvised if, instead of cementing a piece of glass, a thin piece of paper of corresponding size, with a circular aperture, is fastened upon an ordinary slide. The warm stage may be employed for direct observation at higher temperatures.

STAINING BACTERIA.

In examining unstained bacteria the diaphragm is used in order to make the structure-picture clear; but small objects and particles, the size of bacteria, imbedded in the tissue made visible in the structure-picture, are hidden by the shadows of the structure-picture. If these particles are stained, and if they are of a certain size, they will be visible in spite of the shadow; but under this size, notwithstanding their color, they are concealed by the

shadows. Therefore it is desirable to stain the bacteria and to so arrange the light that the structure-picture does not further interfere, and that the color-picture, as pure as possible, be presented to the observer. Koch (loc. cit.) succeeded in producing this isolation of the color-picture by removal of the diaphragm. In this way so weak a structure-picture was produced that the minutest particles of the color-picture became distinct. In the stronger structure-pictures, after the removal of the diaphragm, he used a condenser which threw so intense a cone of rays upon the object that the diffraction appearances were entirely

avoided.

With such a method of illumination, in which the preparation is permeated in all directions by the penetrating rays, only those elements remain visible which produce an absorption of the rays on account of their staining. Further, Abbé in this way has shown that, although the illumination in name remains central, yet the important advantages of the oblique illumination are obtained through the cooperation of the rays passing at a greater inclination to the axis of the microscope. On account of this co-operation of the oblique rays for the isolation of the color-picture, and for the complete development of the capacity for differentiation of the oil-immersion objective necessary for this, the condenser must furnish a cone of light of a size at least equal to the aperture of the objective, which is made, according to Stephenson, with a large angle of aperture. This is as yet attained, in a manner that fulfills all the requirements, only by an Abbé condenser. In order that the structure-picture can be brought out in spite of this condenser, an arrangement for interposing a