Or perhaps he is given a rusty knife, or a perfectly clean hatchet, and is asked to say if there is blood on it. And when he comes into court he is expected to tell the jury whether the blood is human or animal, how old it is, was it spilled from a living blood vessel, and in what part of the body was this blood vessel. Take an actual case. Years ago a celebrated murder was committed in Eltham, and in the report of Dr. Letheby, the analyst, is the following note:-- "On the evening of May 3rd I received from Mr. Mulvaney" (of the police) "a brown paper parcel containing a pair of dark trousers, a man's shirt, and a man's wide-awake hat. On the following evening I received from Mr. Mulvaney a brown paper parcel containing a lock of hair, a pair of men's boots, and a plasterer's hammer." These were all very dirty, but that did not prevent the analyst from finding a number of blood stains and hairs, and giving valuable and decisive evidence at the trial. What the analyst first does, when he receives such an article as a pair of trousers, is to scrutinise every inch of its surface with a magnifying glass. If he finds a little lump of dark-coloured stuff he scrapes it off and puts it into a watch glass. If he discovers merely a dark stain, he cuts out the piece of cloth and puts it into a small quantity of distilled water. [Illustration: HUMAN BLOOD MAGNIFIED 400 TIMES.] Now he has to find out whether the suspicious-looking thing is really blood, or whether it is merely red paint, or logwood, or cochineal, or madder, or iron-mould. There are three ways of doing this, and he nearly always utilises them all. [Illustration: PIG'S BLOOD MAGNIFIED MANY TIMES.] First, there is the marvellous spectroscope test. This test will reveal the presence of the minutest trace of blood, and it is practically infallible. It depends on the curious property, possessed by nearly all bodies, of absorbing certain parts of the light that passes through them. Sunlight passing through a prism is split up into the familiar seven colours of the rainbow. But if a little blood dissolved in water is placed in a glass tube, and if the light is made to pass through it on its way to the prism, the blood takes something out of it; for now among the seven bright colours are seen two dark bands near the middle of the yellow ray. Nothing but blood gives these two bands in that particular place, with the exception of two or three substances that are not likely to be found on criminals' clothes. These are cochineal, mixed with certain chemicals, hot purpurin sulphuric acid, and the red dye of the banana-eater.