MICH ILIMACKINAC MICROSCOPE. 



803 



the northern part of the United States, and wholly 

 within the territory of these states. It has the Michi- 

 gan Territory on the east, Indiana on the south, and 

 is connected on the north-east with lake Huron, by 

 the strait of Mackiuac. Its length is nearly three 

 hundred miles, its breadth about sixty miles, and its 

 average depth about 900 feet. The distance from 

 the southern extremity to the Mississippi is 161 miles. 

 Lon. 84 40' to 87 8' W.; lat. 41 15' to 45 35' N. 

 It contains, according to Hutchins, 10,868,000 acres. 

 The waters are clear and wholesome, and contain 

 many kinds of fish. In the north-west part there are 

 two large bays, Noquet's and Green. 



MICHILIMACKINAC, orMACKINAC ; a post- 

 town and military post in Michigan. It is situated 

 upon an island in the strait connecting lake Huron 

 and lake Michigan ; the best authorities now give to 

 the town and island the name Mackinac, and to the 

 county of which the town is the capital, and the 

 strait in which it is situated, that of Mackilimackinac. 

 The common pronunciation is Mack-i-naw, and the 

 name is not unfrequently written in this manner. The 

 island is about nine miles in circuit. The population 

 of the county, in 1830, was 877. It is much resorted 

 to by fur traders, and during the summer is visited by 

 thousands of Indians, on their way to Drummond's 

 island. On a cliff above the town is the fort. Lon. 

 84 40' W. ; lat. 45 54' N. 



MICHILIMACKINAC, STRAITS OF ; a channel 

 connecting lake Michigan with lake Huron, forty 

 miles long from east to west, and four miles wide in 

 the narrowest part. 



MICKLE, WILLIAM JULIUS, an English poet, the 

 son of a Presbyterian clergyman, was born in Dum- 

 friesshire, Scotland, in 1734, and received his educa- 

 tion at Edinburgh. At first he engaged in business 

 as a brewer, but not succeeding, he devoted himself 

 to literature, and removing to London, was noticed by 

 lord Lyttleton. In 1765, he was employed as cor- 

 rector of the press in the Clarendon printing-office at 

 Oxford, where he published a poem entitled the Con- 

 cubine, in imitation of Spenser, republished with the 

 title of Sir Martyn. He afterwards edited Pearch's 

 Collection of Poems, 4 vols. supplementary to that of 

 Dodsley. In 1775, appeared his principal produc- 

 tion, a translation of the Lusiad of Camoens. Pre 

 fixed to the poem is a historical and critical Introduc- 

 tion, including a life of Camoens ; and the work it- 

 self is executed in a manner highly creditable to the 

 talents of the translator. In 1778, Mr Mickle accom- 

 panied commodore Johnson as his secretary on a 

 mission to Lisbon ; and died in 1788. His poetica] 

 works were published collectively, in 3 vols. 8vo, 

 1807, with a biographical memoir. 



MICROMETER ; an instrument fitted to tele- 

 scopes in the focus of the object-glass, for measuring 

 small angles or distances, as the apparent diameters 

 of the planets, &c. Various forms have been given 

 to this instrument by different authors, and various 

 claims have been urged for the honour of the inven- 

 tion. It seems, however, to belong to Gascoigne, an 

 Englishman, though it is doubtful whether 1 1 uy gens 

 did not also invent the one which he used, without 

 any knowledge of that of the former. Under all the 

 forms of this instrument, the principle of operation i: 

 the same, which is, that it moves a fine wire paralle 

 to itself, in the plane of the picture of an object 

 formed in the focus of the telescope ; and with such 

 accuracy as to measure with the greatest precisioi 

 its perpendicular distance from a fixed wire in the 

 same plane, by which means the apparent diameter 

 of the planets, and other small angles, are exactly 

 determined. This may be illustrated as follows : 

 Let a planet be viewed through a telescope, and 

 when the parallel wires are opened to such a distance 



as to appear to touch exactly the two opposite 

 ixtremities of the disc of the planet, it is obvious that 

 he perpendicular distance between the wires is then 

 equal to the diameter of the object in the focus of the 

 object glass. 



MICROSCOPE. The history of the microscope 

 s veiled in considerable obscurity, and among the 

 noderns the discovery of this instrument has been 

 laimed by several individuals. The ancients appear 

 o have been acquainted with it in one of its forms ; 

 or Seneca says, " Letters, though minute and ob- 

 scure, appear larger and clearer through a glass 

 3ubble filled with water." In the middle ages this 

 knowledge was lost. The invention of the modern 

 nstrument is attributed by the celebrated Dutch 

 mathematician Huygens, to a countryman of his, 

 named Drebell, who constructed them about 1621, or 

 thirty-one years after the invention of the telescope. 

 Borelli attributes it to Jansen, the reputed contriver 

 of the telescope ; Viviani to Galileo. The first micro- 

 scope, consisting of two double convex lenses, seems 

 to have been made by F. Fontana, a Neapolitan, who 

 dates his invention from 1618. The numerous forms 

 of microscopes may be included under the heads of 

 single, compound refracting and compound reflecting 

 microscopes. The theory of the single microscope may 

 be thus explained. We all know that at a small dis- 

 tance we see more distinctly than at a large. If we 

 look at two men, one 200 feet distant, the other 100 

 feet, the former will appear only half the height of 

 the latter, or the angle which the latter subtends to 

 the eye of the observer will be twice that subtended 

 by the former. Hence we must conclude, that the 

 nearer we can bring an object to the eye, the larger 

 it will appear. Now if to render the parts of a 

 minute object distinguishable, we bring it very near 

 the eye (suppose within one or two inches), it will 

 become very indistinct and confused, in consequence 

 of the great divergence of the rays of light from the 

 object, and the power of the crystalline lens of the eye 

 not being sufficient to collect the rays whereby an 

 image of the object may be formed on the retina at 

 the proper distance on the back of the eye. Now if 

 we employ a single microscope, which consists of a 

 convex lens usually made of glass (though any other 

 transparent substance would have the same power in 

 a^reater or less degree), and mounted in a brass 

 setting, and place it between the object and the eye, 

 the former being in the focus of the glass, the diverg- 

 ing rays from the object will be refracted and rendered 

 parallel by the lens, and thus we shall obtain a dis- 

 tinct and near view of the object. The increase or 

 apparent magnitude obtained by the employment of 

 lenses, is proportioned to the difference of the dis- 

 tance of an object from the lens and the distance 

 when seen without its assistance. This latter distance 

 (the distance of distinct vision of minute objects with 

 the naked eye) varies in different persons, and at dif- 

 ferent periods of life. Some measure therefore must be 

 assumed as a standard, before we can express the am- 

 plifying power of a lens so as mutually to have the same 

 idea of the magnitude of an object. Some authors 

 adopt ten inches as the standard of the focus of the eye, 

 under ordinary circumstances, and its decimal charac- 

 ter makes it a convenient multiplier or divisor. With 

 this decimal standard we can determine the magnify- 

 ing power of lenses of any focal length, or formed of 

 any substance (media). Thus if we have a lens 

 which requires for distinct vision the object to be one 

 inch from its centre (in a double convex), we must 

 divide the standard ten by one which will give ten 



as the magnifying power. If the lens require the 

 object to be l-25th of an inch distant, its magnifying 



power will be 250. We have called the magnifying 



power in the first instance ten, because the length of 

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