From Hooke’s description I gather that he also introduced the ball-and-socket movement into the construction of the body of his instrument. This has found many imitators since his day; some of them have gone so far as to claim the invention as one quite new. For small accessories, where the leverage need not be considered, the ball-and-socket has proved convenient enough; but not, however, if applied to the stand of the microscope. Hooke, in his early work, expressed dissatisfaction with the English-made lenses he had in use. He complains of the “apertures of the object-glasses, which are so small that very few rays are admitted; none will admit a sufficient number of rays to magnifie the object beyond a determinate bigness.” So we may take it that he thus early discovered the great importance of an increase in the aperture of his microscope. Other improvements of importance were made, and he was the first to describe a useful method of estimating the magnifying power of his lenses, and the difficulty of distinguishing between a prominence and a depression in the object under investigation, which he was made more fully aware of when preparing drawings for the illustration of his “Micrographia Illustrata”; this would be in 1664, if not earlier. His book created no little sensation on its first appearance, and it soon became scarce. Hooke (says Mr. Mayall) “must undoubtedly be credited with the first suggestion of immersion lenses.” Nevertheless, in his “Lectures and Collections,” published in 1676, he appears to be no longer enthusiastic over his double microscope, and once more he reverts to the simpler instrument of his earlier days. Whether this change of opinion was due to the publication of Leeuwenhoek’s observations with his simple microscopes it is impossible to say.