212 THE MICROSCOPE AND ITS LESSONS. 



central offspring, which will take the place of its parent, 

 with a third generation concealed in it, one taking the 

 place of the other when it has accomplished its work. 

 This book does not pretend to approach a treatise on 

 physiology, and so all that is necessary, in thinking of the 

 important part of life which the red corpuscles play in 

 bodily health, is to let you see upon what little things 

 much of our happiness depends. Observe they are 

 flattened discs. I have told you their diameter is only 

 ^5 a oTJ P art f an i ncn - The circle upon which they are 

 spread on our glass slip measures half an inch in diameter. 

 It is ten times larger in circumference- than a pin's head, 

 and three millions of these red blood-discs, remember, 

 will lie in that space. 



These blood-discs are very indestructible, and may be 

 extracted from the blood-stains on weapons upon which 

 the blood has remained for many years ; hence the value 

 of the microscope in criminal investigation. The scarlet 

 hue is due to iron, and from the dead bodies of their 

 friends, when cremated, our French neighbours sometimes 

 find iron enough to make a mourning-ring. 



Every time your heart beats it tolls the knell of twenty 

 millions of these little organisms.* You may naturally 

 ask, How is it possible to measure these infinitely small 

 bodies, or to count such vast numbers ? 



The same question might fairly have been asked as 

 we looked at the many thousand lenses of the gnat, or 

 those of beetles. It were impossible to measure or to 

 count them, but we have a micrographic measurer, called 

 a micrometer, that is, a measurer of little things a photo- 

 graph, on which are about a thousand minute divisions, 

 each numbered, and by laying this over the object, and 

 counting or measuring a given number in one division, 



* Dr. Laweon, iii " Popular Physiology." 



