AND TITS BEAKINGS ON PATHOLOGY 369 
rim of fluid that formed round about its margin was so narrow as not to measure 
a quarter of the diameter of the field of the microscope even when the highest 
magnifying power was used. In other words, 1-50th minim was disposed in 
a thin uniform layer of the exact size of the covering glass. Hence the number 
of bacteria under the glass slip, that is to say in I-50th minim, was equal to 
the number of the bacteria in a field of the microscope multipled by the number 
of times the area of that field went into the area of the covering glass. The 
micrometer gave the diameter of the field in thousandths of an inch; and the 
covering glass measured 500-thousandths of an inch across: and the areas of 
the circles were of course proportioned to the squares of those diameters. All 
that was needful, therefore, in order to enable me to calculate the number of 
bacteria in 1-50th minim, was to form a fair estimate of the number of bacteria 
per field, and this was done by counting the organisms in a considerable number 
of fields and taking the average.* 
It so happened that two kinds of bacteria were seen under the microscope, 
one motionless, with the characters of Bacterium lactis, the other much less 
numerous, with longer segments and in active movement. Asa rule, on examin- 
ing milk which is undergoing the lactic fermentation but is still fluid, the 
Bacterium lactis is alone discoverable ; but in this instance we had ocular proof 
of the admixture of another sort, though in much smaller numbers. As the 
result of the estimate which I made of the number of bacteria present in every 
1-50th minim, I found it necessary to dilute the milk with no less than a million 
parts of boiled water in order that every 1-10o0th minim should contain on the 
average a single bacterium.? This having been done, 1-rooth minim of the 
* I found great advantage from placing the circular covering glass (with the liquid) on the under 
surface of a plate of thin glass, large enough to cover a chamber excavated in a thick glass plate. A 
drop of water sufficed to fix the edges of the thin plate of glass down upon the thick, and served also 
to prevent any evaporation taking place from the liquid to be examined into the air-chamber, so that 
the liquid was maintained of constant quantity. At the same time, the object being on the under side 
of the thin plate of glass, an immersion lens could be used for examining it to its extreme limits without 
the inconvenience of the liability of the drop of water under the object glass mingling with the liquid 
examined, as would have been the case had the object been on the upper side of the glass 
plate. 
* The dilution with this enormous proportion of water was readily done by dividing the process 
into two stages. First, by means of the small syringe (figured at p. 365) with purified nozzle, 1-100th 
minim of the milk was added to 200 minims of boiled water, which had been measured into a purified 
glass by means of a pure graduated pipette connected with a syringe, the end of the pipette adapted 
to the syringe, having been stuffed with cotton-wool before purification in the hot box, soas to filter air 
that passed into the pipette from the syringe. When the milk had become thoroughly diffused through 
the water, as indicated by uniform opalescence (the diffusion having been promoted by stirring rapidly 
with a pure glass rod), a minim of this first dilution was transferred by means of another and smaller 
pure syringe-pipette into a second purified glass, which had been charged with 50 minims of boiled water. 
I need hardly add that the utmost care was used to avoid more than momentary exposure of the 
liquids manipulated, and of the purified pipettes or other apparatus employed. 
LISTER I Bb 
