After this period, the mind, either satisfied with the discoveries already made, which will be particularly described hereafter, or tired by its own exertions, sought for repose in other pursuits; so that for several years this instrument was again, in some measure, laid aside. In 1770, Dr. Hill[19] published a treatise, in which he endeavoured to explain the construction of timber by the microscope, and shew the number, the nature, and office of its several parts, their various arrangements and proportions in the different kinds; and point out a way of judging, from the structure of trees, the uses they will best serve in the affairs of life. So important a subject soon revived the ardor for microscopic pursuits, which seems to have been increasing ever since. About the same time, my father contrived an instrument for cutting the transverse sections of wood, in order that the texture thereof might be rendered more visible in the microscope, and consequently be better understood; this instrument was afterwards improved by Mr. Cumming. Another instrument for the same purpose, more certain in its effects, and more easily managed, is represented in Fig. 1. Plate IX; and will be described in one of the following chapters. Dr. Hill and Mr. Custance now endeavoured to bring back the microscope nearer to the old standard, to increase the field by the multiplication of the eye glasses, and to augment the light on the object, by condensing lenses; and in this they happily succeeded: Mr. Custance was unrivalled in his dexterity in preparing, and accuracy in cutting thin transverse sections of wood. [19] Dr. Hill on the Construction of Timber. In 1771, my father published a fourth edition of his Micrographia, in which he described the principal inventions then in use; particularly a contrivance of his own, for applying the solar microscope to the camera obscura, and illuminating it at night by a lamp, by which means a picture of microscopic objects might be exhibited in winter evenings. It appears[20] from the testimony of M. Æpinus, that Dr. Lieberkühn had considerably improved the solar microscope, by adapting it to view opake objects. This contrivance was by some means lost. The knowledge, however, that such an effect had been produced, led Æpinus to attend to the subject himself, in which he in some measure succeeded, and would, no doubt, have brought it to perfection, if he had increased the size of his illuminating mirror. Some further improvements were made on this instrument by M. Ziehr; but the most perfect instrument of the kind, is that of Mr. B. Martin, who published an account of it in the year 1774.[21] The common solar microscope does not shew the surface of any object, whereas the opake solar microscope not only magnifies the object, but exhibits on a screen an expanded picture of its surface, with all its colours, in a most beautiful manner. [20] Priestley’s Hist. of Optics, p. 743.