From our remarks concerning the sizes of the red corpuscles in the blood of man and various animals, one might be excused from thinking that it would be quite easy for anyone with a little experience to recognise human blood. As a matter of fact it is very difficult, it is always a doubtful matter to rely upon size alone. We have mentioned blood crystals and these give us a slightly better clue to the origin of the blood, for the blood crystals of different animals vary in shape, far more than their corpuscles. Those of the guinea-pig, for example, are little four sided pyramids; those of the mouse, eight sided and so on. Some apes, however, have blood crystals very similar to those of man, so similar that the two may be confused. The microscopist who is compelled to give an opinion concerning the origin of a sample of blood, especially blood which is some days old, is faced with no light task. Without the assistance of the microscope, medical men would never have discovered the cause of many of these insect-borne diseases, they could never have traced the development of the blood parasites in the bodies of insects. For many years it was thought that the disease malaria was caused by the damp air of low-lying land, in fact the word malaria is derived from two Italian words meaning bad air. It was not till the advent of the microscope that the true cause of the disease was learned and the discovery was made that the only connection between the malady and dampness is that the mosquitoes which carry the disease can only thrive in damp situations. Medical work with the microscope also entails a study of external and internal parasites of man other than those which infest the blood. Some of the larger human parasites we know only too well, they force their unpleasant attentions upon us from time to time, but fortunately they are not as a rule serious and are soon got rid of. The cleanest of us, in these days of universal travel, cannot avoid the visitations of the lively flea, he is at worst an annoying companion, but he has a cousin who is responsible for the passing of the germs of the dreaded plague from rats to man. Plague long remained a mystery, which, without the help of the microscope, would probably have remained unsolved to this day. Many of these apparently harmless blood-sucking insects may prove to be disease carriers, as medical knowledge and microscopic investigation is brought to bear upon them. The majority of the smaller external parasites are beyond the reach of the amateur microscopist. A number of course are available to medical students, but we write for the ordinary enthusiast and not for the specialist. Many interesting little mites, closely allied to the cheese-mites, cause skin diseases and the creatures themselves as well as their tunnels in the skin are interesting.