Now we may perform another operation upon ourselves, rather more painful than the last one but not very serious. We wish to examine some human blood, so we tie a handkerchief tightly round one of our fingers and with a clean needle--be sure that it is clean--make a puncture in the finger tip. The handkerchief bandage will prevent our feeling any pain. We must put the drop of blood we have obtained in the centre of a clean slide and examine it under the microscope. While the blood is still liquid, we shall see a number of circular discs floating about, they are very small being only 1/3200 inch in diameter. The centre of each disc appears darker than the rim, but this darker shade is only apparent. Using a term we introduced in our chapter on the lens, we may call the discs double concave. Now let us watch our objects for a moment and we shall notice that they begin to arrange themselves in chains, they appear like a number of draughtsmen placed one upon the other, our drop of blood is now beginning to clot. A simple experiment may be attempted at this stage; we must run a drop of water on to our little blood discs and watch carefully what happens. We shall see that they change their shape gradually, from being double concave they become flat sided, and this is not all, for they continue to change till both sides bulge outwards and they may be described as double convex. These little discs are known as red blood corpuscles. Their shape is some indication of the animal to which they belong; those of man, as we have seen, are circular and so are those of most of the higher animals, except the camel tribe, which has oval, red blood corpuscles. Birds, reptiles and fishes have corpuscles agreeing in shape with those of the camel. In size there is a great deal of difference between the corpuscles of various animals; a member of the deer family has the smallest and a creature related to our Newts, called Proteus, has the largest. The size of the blood corpuscles bears no relation to the size of the animal to which they belong, those of the frog measure 1/1108 inch, or nearly three times the size of the red blood corpuscles of man. Without much difficulty it should be possible to obtain other samples of blood and the red corpuscles should always be examined, needless to say the blood should always be in a fresh condition. In addition to the red corpuscles we may notice a few smaller circular bodies, they are the white corpuscles. If we have any difficulty in finding them in our own blood, we can examine another specimen of frog’s blood, in which they are more easily seen. It has been calculated that in a cubic inch of blood from a healthy human being there are eighty millions of red and a quarter of a million of white blood corpuscles. Although there is such an enormous difference in the sizes of red corpuscles from various animals, the white corpuscles are remarkably constant, measuring about 1/3000 inch in warm-blooded