On Single Microscopes.
A Single Microscope, is an instrument in which only a single lens or mirror is employed for the purpose of magnifying objects. If the instrument is furnished with mirrors or lenses for illuminating the object, or with any other apparatus, however complicated, it still conies under the denomination of a single microscope.
One of the simplest of all single microscopes is a plano-convex, or a double convex lens of glass, fitted into a rim of brass furnished with a handle The ob ject is then held in the interior focus of this lens, and appears magnified in proportion to the smallness of its focal length.
Microscopes of this kind were first successfully ap plied to the examination of natural objects by the cele brated Anthony Van Leewenhoek They consisted of a small double convex lens, carefully ground and po lished, and inserted between two thin plates of metal, pierced with a hole smaller than the diameter of the lens, and afterwards rivetted. The object was placed upon a silver point or needle, which, by the agency of screws, could be turned in all directions, and placed at any requisite distance from the lens. To this needle the objects were fixed with glue when they were solid ; and when they were fluid, he put them on a thin film of mica, or brown glass, which was afterwards attached to the needle by glue In all Lcewenhoek's micro scopes the lenses had not a very high magnilying power ; and there is reason to believe, that most of his disco veries were made more from the dis.inrtness arising from the accurate figure and good polish of his lenses, than from the greatness of their power. Of the twenty six microscopes which he presented to the Royal So ciety, only one had a focal length so small as of an inch, and all the rest were below half an inch in focal length.
Dr Hooke appears to have been the first who sub stituted glass globules in place of convex lenses. In the preface to his Micrographia, published in 1665, he describes the method of making them. A small rod or strip of thin and good window glass is drawn out into fine threads, either by holding it in the flame of a candle, or a lamp with spirit of wine or good oil. The end of one of these threads is then held in the flame till it runs into a small drop or globule of the required size. When the globule has cooled, it is fixed
upon a thin plate of brass or silver, so that the centre of it is directly over the centre of a very small hole in the metallic plate. In this way, none of the rays of light that issue from an object placed in its focus pass through the part of the globule where it is joined to the glass thread. Dr. Hooke also fixed the globules with sealing wax to the end of a stick, so that the threads stood up wards, and he ground off the ends of the threads, and polished them.
The art of making microscopic globules of glass was brought to great perfection by Father di Torre of Na ples. Having formed glass globules by the method already described, he separated the ball of glass from the thread by the sharp edge of a piece of flint. When a great number of globules were made in this manner, he next proceeded to give them a complete spherical form, by melting them a second time. For this pur pose, he made use of a piece of tripoli about four or five inches long, and three or four inches thick. After it had been calcined, by surrounding it with charcoal nearly red hot, and allowed to grow quite cold, several small hemispherical cavities of different diameters are made on the flat side of it, and arc nicely polished and neatly rounded at the edge. The small glass globules, after being carefully cleaned, are placed in the cavities of the tripoli by a pair of delicate nippers. The ex tremity of the flame of a blow-pipe is then directed to wards the globules, which assume a perfectly spherical form when brought to a fluid state, and never adhere to the tripoli. When the globule is cold, it is cleaned by rubbing it between two pieces of paper, and is then set in a brass cap for the purpose of trying its figure, by viewing an object through it. It sometimes hap pened, that in damp weather onlyfour or five globules out of forty were lit for use. In the year 1'761, four of Father di Torre's microscopes were sent in a present to the Royal Society. One of them had a diameter of .Ath of an inch, two of them a diameter of part of an inch, and the fourth a diameter of only the of an inch. Mr. Baker, who examined these globules with great care, could not make use of the smallest of them, and considered them as more curious than useful.