Dr. Wollaston employed a new form of combination in a microscope constructed for his own use, and by which “he was able to see distinctly the finest markings upon the scales of the _Lepisma_ and _Podura_, and upon those of the gnat’s wing.” His doublet is still employed, and to which I shall refer under “Simple Microscopes.” [Illustration: Fig. 3_a_.--Sir David Brewster’s Microscope, of the early part of the century, recently presented to the British Museum.] CHAPTER I. Elementary Optics. Value of Inductive Science--Light: Its Propagation, Refraction, Reflection--Spherical and Chromatic Aberrations--Human Eye, formation of Images of External Objects in--Visual Angle increased--Abbe’s Theory of Microscopic Vision. The advances made in physics and mechanics during the 17th and 18th centuries fairly opened the way to the attainment of greater perfection in all optical instruments. This has been particularly exemplified with reference to the invention of the microscope, as briefly sketched out in the previous chapter. Indeed, in the first half of the present century the microscope can scarcely be said to have held a position of importance among the scientific instruments in frequent use. Since then, however, the zoologist and botanist by its aid have laid bare the intimate structure of plants and animals, and thereby have opened up a vast kingdom of minute forms of life previously undreamt of; and in connection with chemistry a new science has been founded, that of bacteriology. For these reasons it will be of importance to the student of microscopy to begin at the beginning, and it will be my endeavour to introduce to his notice such facts in physical optics as are closely associated with the formation of images, and, so to speak, systematise such stepping stones for work hereafter to be accomplished. Elementary principles only will be adduced, and without attempting to involve my readers in intricate mathematical problems, and which for the most part are unnecessary for the attainment of the object in view. I therefore pass at once to the consideration of the propagation of light through certain bodies.