VI. Description of a -Vertu Compounl Microscolie for examining objects of Xatural History • ° The construction both of single and compound mi croscopes has, within the last fifty yeats, been brought to a great degree of perfection ; and, for all the purpo ses of amusement and general obsei vation, these ins'ru meats may be considered as sufficiently perfect. But when we employ the microscope as an instrument of discovery, to examine those phenomena of the world which are beyond the reach of unassisted vision, and when we use it in ascertaining the and physiological structure of plants, insects, and animal culx, we soon find, that a limit, apparently insuperable, is set to the progress of discovery, and that it is only some of the ruder and more palpable functions of these evanescent annuals that we are able to bring under ob servation. Naturalists, indeed, are less acquainted with the organization of the microscop:c world, and the beings by which it is peopled, than astronomers are with those remote systems of the universe which ap pear in the form of nebulR and double stars. It was the improvement of the telescope alone which enabled Dr. Herschel to fix the views of astronomers upon those regions of space, to which, at a former period, their imaginations could scarcely extend; and when the mi croscope shall have received a similar improvement, we may look for discoveries equally interesting, though less stupendous, even in those portions of space which are daily trampled ander the foot of man.
" It is both important and interesting to inquire into the cause of this limitation of microscopical discovery. The construction of single lenses for the simplest form of the instrument, has been brought to great perfection. I have in my possession glasses executed by Mr. Shut tleworth, of the focal length of ,,'„„ and of an inch, which are ground with great accuracy ; and the performance of single lenses has been recently impro ved by Dr. Wollaston, as described in p. 240. We can not, therefore, expect any essential improvement in the single microscope, unless from the discovery of some transparent substance, which, like the diamond, com bines a high refractive power with a low power of dis persion.
" In the combination of single lenses to form the com pound microscope, opticians have likewise arrived at a great degree of perfection. Tne aberration of refran gibility can now be completely removed by a suitable arrangement of the individual lenses ; and every arti fice has been exhausted in suiting the apparatus to the various tastes of purchasers, and to every purpose of popular observation.
" No attempt, however, appears to have been made by opticians to fit up the microscope as an instrument of discovery, to second the labours of the naturalist ill pre paring t! e subjects of his research, and to accommodate the instrument to that particular kind o/ preparativ; which is indispensably necessary fir the preservation and inspection of minute objects.
" In perusing the writings of those n..turalists who have applied the microscope to the examination of mi nute objects, we find, that the most difficult and per plexing part of their labour consisted in preserving and preparing the different insects and substances which they wished to inspect. Small insects instantly shrivel up and lose their natural form as soon as they are kill ed, and the minute parts of plants suffer a similar change from exposure to the air. 'fence Swammerdam and Lyonet killed the insects which they meant to ex amine, by suffocating them either in water, spirit of turpentine, or diluted spirits of wine. The softness and transparency of their parts were thus preserved du ring the process of dissection, and when they were com pletely developed, the insect was allowed to dry before it was presented to the microscope. Its parts were con sequently tontracted, and lost not only their proper shape, but that plumpness, and that freshness of colour which they possessed when alive.
" In the preparation, indeed, of almost every object of natural history that is composed of minute and delicate parts, it must be preserved by immersion in a fluid ; the dissection must often be performed in the same medium ; it must be freed from all adhesive and ex traneous substances, by maceration and ablution in wa ter ; and when it has undergone these operations, it is in a state of perfection for the microscope. Every sub sequent change which it undergoes is highly injurious ; it shrivels and collapses. by being dried ; its natural polish and brilliancy are impaired ; the minute parts, such as the hairs and down, adhere to one another, and the general form of the object, as well as the disposi tion of its individual parts, can no longer be distinctly seen.
64 It is therefore a matter of considerable importance to be able to examine the object when wet, and before it has suffered any of these changes ; and by fitting up the microscope in the following manner, this may be effected without even exposing the object to the air.
44 The object-glass of the compound microscope should have the radius of the immersed surface about nine times the focal distance of the lens, and the side next the eye, about three-fifths of the same distance. This lens should be fixed into its tube with a cement which will resist the action of water or spirits of wine ; and the tube, or the part of it which holds the lens, si,ould have an universal motion, so that the axis of the lens may coincide to the utmost exactness with the axis of the tubes which contain the othe• glasses.