A similar instrument may be made by means of a doubly refracting plate placed at the upper end of a tube. A rectangular aperture is placed before this plate, of such a magnitude that its two images are ex actly in contact when they are parallel to each other. By giving a rotatory motion of 90° to the rectangular aperture, the two apertures coincide in the direction of their length, and we have an arch of 90° to measure the variation. The breadth of the compound aperture will always be A-l-A sin. cp, A being the breadth of the rectangular aperture. and the eP arch described by the aperture reckoned from the longitudinal coincidence of the two images.
The principle of double refraction may be advanta geously employed in the construction of a micrometer for measuring directly the magnitudes of objects of ex treme minuteness. Let it be required, for example,to measure objects whose diameters are between the 3-1-Eth of an inch. Take a plate of rock crystal AB, CD (Plate CCCLXX VI. Fig. 25.) about two inches long, whose thickness AB at one end is ten times greater than its thickness CD at the other, and let the plate be cut out of the crystal, so that the two images of a wire of an inch in diameter will be in accurate con tact, then at the other end CD, the two images of a wire of an inch in diameter will be in accurate contact, and wires, or objects of all intermediate mag nitudes, will he measured by observing the part of the scale between A and C, where their two images are in contact. This scale being two inches long may con sist of a plate of mother-of-pearl attached to the rock crystal, and divided into 400 parts, so that each division MicnoscoPE, from fcoceos., small, and c-zo7reeo, to see. is the name of a well known optical instrument, for seeing or magnifying minute objects.
The invention of the microscope, like that of the te lescope, has been claimed fur more than one indivi dual.* Peter Borellus, in his work lie Vero Tele6copii inventore, published at the Hague in 1655, has addu ced a good deal of evidence connected with the inven tion of the telescope and the microscope. He brings for ward five different testimonies, and a letter from William Borcel, envoy from the States of Holland, which throw considerable light on the subject. Boreel was intimately acquainted with Zacharias Jansen, and had frequently been in his father's shop. He had often heard that the
Jansens were the inventors of the microscope ; and hav ing been in England in 1619, he saw in the hands of his friend Cornelius Drebell, the very same microscope which Zecharius Jansen and his lather had presented to Prince Maurice, and Albert, archduke of Austria. This instrument was six feet long, consisting of a tube of gilt copper, an inch in diameter, supported by thin brass pillars in the shape of dolphins, on a base of ebony, for the purpose of holding the objects to be examined. There is reason to think, that this was nothing more than a telescope converted into a compound micro scope. Cornelius Drebell, therefore, who has common ly been considered as the inventor of the microscope, appears to have derived this honour from the acciden tal circumstance of his having exhibited the micro scope made by Janstn ; and as he was a favourite at the court of James VI. where he lived some time, this opinion may have proceeded, not only from his own arrogance, but from the influence of royal favour.
Vivtani expressly informs us, in his life of Galileo, that this great man was led to the construction of the microscope from that of the telescope ; and that, in the year 1612 he actually sent a microscope to Sigismund, King of Poland. Dissatisfied with the performance of this instrument. he appears, from his letters, to have been much occupied about 1624 in bringing it to per fection, but we have no information respecting the re sult of his labours.
As there is no reason to believe that the microscopes invented by Jansen consisted of two convex lenses, like those now in use. the honour of this improvement seems to be due to M. Fontana, a Neapolitan, who first described it in his work entitled Ave Terrestrium ct Celestium ObRervationes, A"eap. 1646. Fontana distinctly assumes the merit of this improvement, and as no other person has laid claim to it, we agree with Montucla in allowing him the possession of this honour t In pursuing this subject, we shall confine ourselves solely to the description of microscopes, supposing that the reader is acquainted with the principles of their con struction, which fall to be explained under the article OPTICS. We shall treat, 1st, On Single Microscopes ; 2dly, On Compound Microscopes ; 3dly, On the Solar Microscope ; 4thly, On the Lucernal Microscope ; and, 5thly, Or. Microscopic Objects.