THE MICROSCOPE.

INTRODUCTION.

No one who attentively examines the progress of any department of Science, save such as are (like Mathematics or Metaphysics) of a purely abstract character, can fail to perceive how much it is dependent upon the perfection of its instruments. There are few instances, in fact, in which the invention of a new instrument, or the improvement of an old one, has not given a fresh stimulus to investigation; even where it has done no more than afford that degree of precision to the results of inquiries already in progress, which alone could enable them to be made available as data for philosophical reasoning. But there are many cases in which such inventions or improvements have opened out entirely new paths of scientific research, leading to fertile fields of investigation whose very existence had been previously unknown, to rich mines of discovery whose treasures had lain uncared for because entirely unsuspected. A few examples of this general truth may not be inappropriate, by way of preface to the brief notice which it is intended to give in the present Introduction, of the most important epochs in the history, as well of the Microscope itself, as of its application to the purposes of scientific research.

Thus in taking a retrospective survey of the history of Astronomy, we find that every great advance in our knowledge of the Celestial Universe, has been preceded by improvements, either in those instruments for measuring space and time, by which the places of the Heavenly Bodies are determined, the rate of their movements estimated, and a basis for the computation of their distances ascertained; or, again, in the telescope, by which our power of sight is so wonderfully augmented, that we are enabled, when gazing through it into the unfathomable depths of space, to take cognizance of world beyond world and system beyond system, whose remoteness cannot be expressed by any form of words that shall convey a distinct idea to the mind, and to bring the members of our own group within such visual proximity to

ourselves, that we can scrutinize their appearance nearly as well as if they had actually been brought a thousand times nearer to us. For it was the increased precision of celestial observations on the places and movements of the Planets, which furnished the data whereon Kepler was enabled to base his statement of the laws of their motion. It was the application of the pendulum to the measurement of short intervals of time, that enabled Galileo to ascertain the law of Falling Bodies. And it was not until the precise measurement of a degree upon the surface of the Earth had furnished the means of determining both its own diameter and its distance from the Moon, that Newton was enabled to verify and establish his grand conception, of the identity of that force which deflects the planets from a rectilineal course into elliptical orbits, with that which draws a stone to the ground; and thus to establish that Law of Universal Gravitation, which still remains the most comprehensive, as well as the most simple, of all the generalizations, within which the intellect of man has been able to comprehend the phenomena of Nature. So, again, it was only when the elder Herschel had developed new powers in the telescope, that Sidereal Astronomy could be pursued with any view much higher than that of mapping the distribution of the stars in the celestial sphere; and the present state of our knowledge of double, triple, and other combinations of stars, with their mutually adjusted movements, of the multiform clusters of luminous points which seem like repetitions of our own firmament in remote depths of space, and of those nebulous films which may be conceived to be new worlds and systems in process of formation, has only been rendered attainable by the improvements which have been subsequently made in the construction of that majestic instrument.

If we glance at the mode in which the fabric of our existing Chemistry has been upreared, we at once see that it could not have attained its present elevation and stability, but for the instrumentality of the perfected balance; by whose unerring indications it was that the first decisive blow was given to the old "phlogistic" theory, that the foundation was laid for true ideas of chemical combination, that the Laws of that Combination were determined, and that the Combining Equivalents of different elementary substances were ascertained; and by whose means alone can any of those analytical researches be prosecuted, which are not only daily adding to our knowledge of the composition of the bodies which surround us, and suggesting the most important applications of that knowledge to almost every department of the Arts of Life, but which are preparing a broad and secure foundation for a loftier and more comprehensive system of Chemical Philosophy.

So, again, the balance of torsion, the ingenious invention of Cavendish and Coulomb, enables the Physical philosopher not merely to render sensible, but to subject to precise measurement

and subdivision, degrees of force that are far too feeble to affect the nicest balance of the ordinary construction, even if it were possible to bring them to act upon it; and strange as it may seem, it has been in such a balance that the Earth itself has been weighed, and that a basis has been thus afforded for the computation of the weights of the different Planets and even of the Sun; whilst in the opposite direction it is employed to furnish those data in regard to the intensity of the electric and magnetic forces, on which alone can any valid theory of their operation be constructed.

The galvanometer, again, in which the minutest Electric disturbances are rendered sensible by the deflection of the magnetic needle, has not only brought to light a vast class of most interesting electric changes which were previously unsuspected (one of the most remarkable of these being the existence of electric currents in the nerves of living animals, first ascertained by M. du Bois-Reymond), but has enabled those changes to be estimated with a marvellous amount of exactness; thus furnishing to observations made by its means, a precision which is quite unattainable in any other mode, and which is absolutely essential to the establishment of any valid theory of electric action. And this same instrument is scarcely less valuable, as serving, by a particular modification of it, for the detection and estimation of changes of Temperature far too minute to be measured by the ordinary thermometer; thus affording the requisite means of exactness to observation, in a department of science to which at first sight it appeared to have no relation.

"What an important influence," says Sir John Herschel, "may be exercised over the progress of a single branch of science, by the invention of a ready and convenient mode of executing a definite measurement, and the construction and common introduction of an instrument adapted for it, cannot be better exemplified than by the instance of the reflecting goniometer; this simple, cheap, and portable little instrument has changed the whole face of Mineralogy, and given it all the characters of one of the exact sciences."

Of all the instruments which have been yet applied to scientific research, there is perhaps not one which has undergone such important improvements within so brief a space of time, as the Microscope has received during the second quarter of the present century; and there is certainly none whose use under its improved form has been more largely or more rapidly productive of most valuable results. As an optical instrument, the Microscope is now at least as perfect as the Telescope; for the 6-feet parabolic speculum of Lord Rosse's gigantic instrument, is not more completely adapted to the Astronomical survey of the heavenly bodies, than the achromatic combination of lenses so minute that they can scarcely be themselves discerned by the unaided eye, is to the scrutiny of the Physiologist into the mysteries of life

and organization. Nor are the revelations of the one less surprising to those who find their greatest charm in novelty, or less interesting to those who apply themselves to the study of their scientific bearings, than are those of the other. The universe which the Microscope brings under our ken, seems as unbounded in its limit as that whose remotest depths the Telescope still vainly attempts to fathom. Wonders as great are disclosed in a speck of whose minuteness the mind can scarcely form any distinct conception, as in the most mysterious of those nebula whose incalculable distance baffles our hopes of attaining a more intimate knowledge of their constitution. And the general doctrines to which the labors of Microscopists are manifestly tending, in regard to the laws of Organization and the nature of Vital Action, seem fully deserving to take rank in comprehensiveness and importance with the highest principles yet attained in Physical or Chemical Science.

As the primary object of this treatise is to promote the use of the Microscope, by explaining its construction, by instructing the learner in the best methods of employing it, and by pointing out the principal directions in which these may be turned to good account, any detailed review of its history would be misplaced. It will suffice to state, that whilst the simple microscope or magnifying-glass was known at a very remote period, the compound microscope,-the powers of which, like those of the telescope, depend upon the combination of two or more lenses,was not invented until about the end of the sixteenth century; the earlier microscopes having been little else than modified telescopes, and the essential distinction between the two not having been at first appreciated. Still, even in the very imperfect form which the instrument originally possessed, the attention of scientific men was early attracted to the Microscope; for it opened to them a field of research altogether new, and promised to add largely to their information concerning the structure of every kind of organized body. The Transactions of the Royal Society contain the most striking evidence of the interest taken in microscopic investigations two centuries ago. Their early volumes, as Mr. Quekett truly remarks, "literally teem" with improvements in the construction of the Microscope, and with discoveries made by its means. The Micrographia of Robert Hooke, published in 1667, was, for its time, a most wonderful production; but this was soon surpassed by the researches of Leeuwenhoek, whose name first appears in the Philosophical Transactions, in the year 1673. That with such imperfect instruments at his command, this accurate and pains-taking observer should have seen so much and so well, as to make it dangerous for any one even now to announce a discovery, without having first consulted his works, in order to see whether some anticipation of it may not be found there, must ever remain a marvel to the microscopist. This is partly to be explained by the fact, that he