Having issued our warning let us see what we can do in the way of microscopic investigation. The easiest subject with which to make a start is the Hay Bacillus, _Bacillus Subtilis_, not because it is the largest of the bacteria by any means, but because it is very easily obtained. Each plant measures about five thousandths of an inch in length, so we shall require a high magnification to examine it. Having obtained a small quantity of hay, we must boil it in water for about three-quarters of an hour and then set it aside for some hours. In due course the water will contain hundreds upon hundreds of bacteria or, speaking more correctly, of bacilli. For our work, we shall require a special kind of microscope slide; instead of the piece of plain glass we have been accustomed to use we must obtain one with a circular portion, hollowed out from the centre. Having done so, we take a clean glass rod and, with it, transfer a drop of the water, containing the bacilli, to the centre of a clean coverslip. Invert the coverslip so that the drop is on the lower surface and place it over the hollow portion of the slide, in such a manner that the drop still remains suspended from the coverslip; this is known as the hanging-drop method and requires some little skill to accomplish satisfactorily. When our slide is prepared, with a magnification of at least one thousand diameters, we may reasonably hope that our trouble will be rewarded. At first we shall probably see nothing. We recall that we had some difficulty in examining starch grains, on account of the fact that they were colourless. This time we are dealing with a far more difficult subject. When our eyes become accustomed to the light, however, we shall be conscious that there is something moving in our drop of water. The Hay Bacillus is one of the moving forms, each individual is furnished with a number of little whips whose lashings enable it to travel through the water. The whips cannot be seen in unstained bacilli; experience, however, tells us that they are there, for all these lowly plants which show movement are seen when stained, to possess the little whips. The process of staining kills the plants so that we cannot see the little whips in action.