Blood, however, changes after it is shed. In stains a few weeks old the colouring matter changes from what is technically called hæmoglobin to methæmoglobin, and, later still, to hæmatin. All of these give different spectra. The analyst has standard spectra already mounted, and he invariably looks at the mounted or standard specimen and the suspected liquid at the same time, placing them side by side, so that a mistake is impossible. All the red colours in the world, in fact, have been tried, and, with the exceptions named above, none of them gives a spectrum like the colouring matter of blood in any of its forms. But though the spectroscope is a certain discoverer of blood, it can draw no distinction between human and animal blood. That duty remains to the microscope. [Illustration: LITTLE LINKS IN THE CHAIN OF EVIDENCE.--THE CORPUSCLES IN THE BLOOD OF DIFFERENT CREATURES. Man. Mouse. Horse. Camel. Toad. Pike. Pheasant. Pigeon.] With the microscope can be seen those red corpuscles which, in some mysterious manner, seize on the oxygen of the air as it passes into the lungs, shoulder it, so to speak, and rush away with it, like so many ants, to the remotest parts of the body. Unfortunately, they can only be seen in blood that has not been very long shed--that is to say, some weeks or months. To see these, the analyst scrapes the little clot from the piece of cloth, or wood, or iron, and places it on a slip of glass; over this he carefully lays the little film called a cover-glass; and then he gently places, at the edge of the latter, the tiniest possible drop of water. This gradually insinuates itself, and soon dissolves the blood clot; and, when the mixture is placed under a microscope magnifying from 300 to 500 diameters, he sees one of several pictures. The various shapes and arrangements taken by these little bodies are illustrated on the following page. Small as they are--it would take 12-1/4 millions to cover a square inch--they have the most peculiar way of behaving, and only the practised eye of the microscopist can recognise them in all their disguises. [Illustration: HUMAN BLOOD CORPUSCLES UNDER THE MICROSCOPE.] Individually, the blood corpuscle is just like a tiny round biscuit, and measures 1/3200 to 1/4000 of an inch across its face. It is these two factors, the shape and measurement, which enable the medical man to say whether the blood is human. The picture above shows how a corpuscle looks under the microscope. Looking at its face, it is like a thick-edged biscuit, with a dark depression in the centre. Some are turned sideways in our illustration. These exist in blood and nothing but blood, so that, when the spectroscope fails, the microscope succeeds.