Many transparent objects are seen best in a weak light; among these we may place the prepared eyes of flies and animalculæ in fluids; the quantity of light from a lamp or candle may be lessened by removing the microscope to a greater distance from them, or it may be more effectually lessened by cutting off a part of the cone of rays that fall on the object, either by placing the cone, as already described with the apparatus to different microscopes, under the stage, or by forming circular apertures of black paper of different sizes, and placing either a large or small one on the reflecting mirror, as occasion may require.
There is an oblique position of the mirrors, and consequently of the light, which is easily acquired by practice, but for which no general rule can be given, that will exhibit an object more beautifully and more distinctly than any other situation, shewing the surface, as well as those parts through which the light is transmitted.
A better view of most objects is obtained by a candle or lamp than by day-light; it is more easy to modify the former than the latter, and to throw it on the object with different degrees of density. From what has been said, the reader will have observed the importance of being able to examine the object in the greatest variety of positions and appearances, which cannot be effected with equal convenience by any microscope, but the improved lucernal.
OF THE PREPARATION OF OBJECTS FOR THE MICROSCOPE.
In the preparation of objects, no man was more successful or more indefatigable than Swammerdam. In minutely anatomizing, in patiently investigating, and in curiously exhibiting the minute wonders of the creation, he stands unrivalled, far exceeding all those that preceded, as well as those which have succeeded him. Deeply impressed and warmly animated by the amazing scenes that he continually discovered, his zeal in pursuit of truth was not to be abated by disappointment, or alarmed by difficulty; and he was never satisfied till he had attained a rational and clear idea of the organization of the object, whose structure he wished to explore; his “Book of Nature,” of which a translation was published by Dr. Hill, is a work of such vast extent of knowledge, and so excellent in execution, as to raise the highest admiration in even a superficial observer.
It is much to be regretted, that we are ignorant of the methods he employed in his investigations. To discover these, the great Boerhaave examined with a scrupulous attention all the letters and manuscripts of Swammerdam, and has communicated the result of his researches, which, though but small, may enable us to form some idea of his immense labours in the field of science.
For dissecting of small insects he had a brass table, which was made by that excellent artist, S. Musschenbroeck; to this table were affixed two brass arms, moveable at pleasure to any part of it. The upper portion of these arms was constructed so as to have a slow vertical motion, by which means the operator could readily alter their height, as he saw most convenient to his purpose; the office of one of these arms was to hold the minute bodies, and that of the other to apply the lens or microscope.
His microscopes or lenses were of various foci, diameters, and sizes, from the least to the greatest, and the best that could be procured in regard to the exactness of the workmanship, and transparency of the substance. His mode was, to begin his observations with the smallest magnifiers, and from thence proceed by degrees to the greatest. Formed by nature, and habituated by experience, he was so incomparably dexterous in the management of these instruments, that he made every observation subservient to the next, and all tend to confirm each other, and complete the description.
His chief art seems to have been in constructing very fine scissars, and giving them an extreme sharpness: these he made use of to cut very minute objects, because they dissected them equally; whereas knives and lancets, let them be ever so fine and sharp, are apt to disorder delicate substances, as in going through them, they generally draw after and displace some of the filaments. His knives, lancets and styles, were so very fine, that he could not see to sharpen them without the assistance of a magnifying glass; but with them he could dissect the intestines of bees with the same accuracy and distinctness that the most celebrated anatomist does those of large animals. He was particularly expert in the management of small glass tubes, which were no thicker than a bristle, and drawn to a very fine point at one end, but thicker at the other. These he made use of to shew and blow up the smallest vessels discovered by the microscope, to trace, distinguish, and separate their courses and communications, or to inject them with very subtil coloured liquors.
He used to suffocate the insects in spirit of wine, in water, or spirit of turpentine, and likewise preserved them for some time in these liquids; by which means he kept the parts from putrefaction, and consequently from collapsing and mixing together; and added to them besides such strength and firmness, as rendered the dissections more easy and agreeable. When he had divided transversely with his fine scissars the little creature he intended to examine, and had carefully noted every thing that appeared without further dissection, he then proceeded to extract the viscera in a very cautious and deliberate manner, with other instruments of great fineness; first taking care to wash away and separate with very fine pencils, the fat with which insects are very plentifully supplied, and which always prejudices the internal parts before it can be extracted. This operation is best performed upon insects while in the nympha state.