Excerpt from The Juvenile Instructor, Vol. 28: February 1, 1893 AT the close of the last paper, a table was given showing the classification of the animal kingdom as accepted by students of zoology today. In our efforts to examine a few types from each of the branches and classes there named, it will be best for us to begin with animals of the simplest structure, and proceed step by step to the study of the more complex. This will necessitate our dealing at first with some of the minutest forms of life; and these can be observed by means of the microscope only. As this instrument will therefore be an indispensable tool for our work, it can not be amiss for us to devote a short time by way of introduction to the microscope itself. The unaided powers of human vision do not enable us to discern objects below a certain size; but the microscope is a device by which the limit of vision is extended so as to embrace objects extremely small. The principle upon which the micro scope is constructed is very simple. Every reader knows that objects look small at a distance, but increase in apparent size as they are approached by the observer, or brought nearer to him. There is a certain distance, however, within which objects cannot be clearly seen; this is called the focal distance of the eye; and though it varies greatly in different persons, the average is about ten inches. In reading, we habitually hold the book at such a dis tance as to afford the clearest vision; if brought nearer, the letters are no longer distinct, but blurred. The medium of vision is light; and light emanates from luminous objects. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.