This third part of the investigation is obviously to prepare specimens for the microscope. "The hanging drop" is a simple plan for securing the organisms for microscopic examination in a more or less natural condition. A hollow ground slide, which is a slide with a shallow depression in it, is taken, and a small ring of vaseline placed round the edge of the depression. Upon the under side of a clean cover-glass is placed a drop of pure water, and this is inoculated with the smallest possible particle taken from one of the colonies of the gelatine plate on the end of a sterilised platinum wire. The cover-glass is then placed upon the ring of vaseline, and the drop hangs into the space of the depression. Thus is obtained a view of the organisms in a freely moving condition, if they happen to be motile bacteria. As a matter of practice the hollow slide may be dispensed with, and an ordinary slide used. [Illustration: DRYING STAGE FOR FIXING FILMS] With regard to staining, it will be undesirable here to dwell at length upon the large number of methods which have been adopted. The "single stain" may be shortly mentioned. It is as follows: A clean cover-glass is taken (cleaned with nitric acid and alcohol, or bichromate of potash and alcohol), and a drop of pure sterilised water placed upon it. This is inoculated with the particle of a colony on the end of a platinum needle, and a scum is produced. The film is now "fixed" by slowly drying it over a flame. When the scum is thus dried, a drop of the selected stain (say gentian-violet) is placed over the scum and allowed to remain for varying periods: _sarcinæ_ about thirty seconds; for many of the bacilli three or four minutes. It is then washed off with clean water, dried, and mounted in Canada balsam. The organisms will now appear under the microscope as violet in colour, and will thus be clearly seen. The "double staining" is adopted when we desire to stain the organisms one colour and the tissue in which they are situated a contrast colour. Some of the details of these methods are mentioned in the Appendix.