G. S. Graham-Smith 
19 
the edges of the cover-glass sealed to the slide all round by means of 
a small quantity of melted vaseline. The specimen was then transferred 
to the stage of a microscope placed within a Nuttall’s microscope 
thermostat, kept at 37° C. Such a preparation can be continuously 
studied for many hours or days. 
This method is applicable to motile as well as non-motile organisms. 
The bacteria cannot lie otherwise than horizontally and in the same 
plane. They are all necessarily in optical contact with the cover-glass 
and are free to grow (horizontally) without the restrictions imposed by 
surrounding them with jelly or the freedom to drift allowed if liquid is 
used. 
2. Growth in the depth. 
In order to study growth in the substance of the agar a small 
quantity of bacterial emulsion was mixed with melted agar at 40° C. 
and the latter poured on a slide. When the agar was just beginning 
to set a cover-glass was placed on it and the specimen finished as in the 
case of surface preparations. 
3. The bacterial emulsion. 
The emulsion of the bacterium to be investigated was invariably 
made from a 12 to IS hour sloped agar culture. In making the emulsion 
sterile tap water was generally used. Even in the young cultures which 
were made use of a certain proportion of the organisms were found to 
be dead, or at any rate incapable of growth under these conditions. 
This was more frequently the case if a portion of the growth from the 
surface of the agar culture was used for making the emulsion. The 
proportion of dead bacteria was much smaller if the growth occurring 
in the water at the bottom of the tube was used. 
4. The choice of an organism for observation. 
The observations were usually made with a Zeiss 4 mm. objective 
and an 8 compensating ocular, artificial light being used for illumination, 
with sub-stage lowered and a narrow diaphragm. 
For satisfactory observations a part of the preparation should be 
sought for where the bacteria are so thinly scattered that only a few 
are within the field at the same time (see Plate VI, Fig. 13). A well 
isolated organism, which looks capable of growth and which is lying in a 
2—2 
