There is one little parasite which we nearly all of us carry without knowing it, and it makes quite an interesting object for the microscope. Its name is _Demodex Folliculorum_ and it dwells in the sweat glands, especially those round about the nose. It is very minute and requires a high magnification for its examination. The adult is worm-like and tapering in the hinder two-thirds of its body, whilst the front third, to which is attached four pairs of short, fleshy legs, is stouter. From the eggs which may be heart or spindle shaped, little six-legged grubs arise; later these grubs change into the eight-legged adults. All the changes take place in the sweat glands, and the creatures live with their heads turned away from the gore of the gland. To observe these strange little creatures it is only necessary to squeeze some of the white congealed perspiration from one of these glands on to a slide, coyer it with a drop of oil or xylol and spread out the object with a needle. By cutting down the light considerably by means of the diaphragm, we shall be able to distinguish all the stages of these little creatures which, unbidden, share our lives. Other parasites come to us in our food and, of these, one of the most dangerous is known as _Trichinella Spiralis_ and gives rise to a disease known as trichinosis. We need not give a precise account of the life history of this interesting worm, it will suffice for our purpose to know that it finds its way into the bodies of human beings, in badly cooked pork or in improperly cured ham or bacon. The little worm is only about 1/25 inch long and, at first, dwells in the intestines of the pig. Each female produces upwards of fifteen thousand young and these pass into the blood of their host and thence to its muscles. Snugly coiled into a spiral between the muscle fibres, each young worm becomes surrounded with a lemon-shaped covering, which is exceedingly resistant. Suppose now a human being should happen to eat such pork, the juices of the stomach would dissolve the pork and also the little case enclosing the parasite. Once set free in the human body, the whole proceeding is repeated and, at last, the result is very severe muscular pains for the victim. The medical man is called upon to study such parasites as _Trichinella_, and they are not so uncommon as one might think. In addition to animal parasites medical science demands a knowledge of many disease bringing fungi. One of the commonest is the fungus responsible for the unpleasant malady called ringworm. But we have written sufficient to show that the general practitioner needs his microscope always by his side. Not only so but very many instruments he uses in his work, though called by other names, are really microscopes adapted for special purposes. CHAPTER XIII THE MICROSCOPE AND AGRICULTURE