32 i DIUECTIONS FOR THE MICROSCOPE HISTORY/ 



gives this account in a letter to his brother Peter, says, that when he 

 was ambassador in England, in 1619, Cornelius Drebell showed him a 

 microscope, which he said was the same that the archduke had given 

 him, and had been made by Jansen himself. The limits of this work 

 will not admit of a description of all the microscopes that have been 

 invented, or the principle and laws by which they are regulated : for 

 much useful and further information on the subject I must therefore 

 refer the reader to the works of Baker, Adams, and others on the mi- 

 croscope, where every information on this head will be found. 



It may nut he amiss, to state clearly and distinctly the method of 

 determining the magnifying powers of glasses employed in single mi- 

 croscopes. 1st. If the focus of a convex lens be at one inch, and 

 the natural sight at eight inches, which is the common standard, an 

 object may be seen through that lens at one inch distant from the eye, 

 and will appear in its diameter eight times larger than to the naked 

 eye. But as the object is magniticd every way equally, in length as 

 well as breadth, we must square this diameter to know really how 

 much it appears enlarged, and we shall then tind that its superficies is 

 indeed magnified sixty-four times. 



cjdly. Suppose a convex lens whose focus is at one-tenth of an inch 

 distance from its centre ; in eight inches there are eighty such tenths 

 of an inch, and therefore an object may be seen through this lens 

 eighty times nearer than it can distinctly by the naked eye. It will 

 consequently appear eighty times longer and eighty times broader than 

 it does to common sight; and as eighty multiplied by eighty makes 

 six thousand and four hundred, so many times it really appears mag- 

 nified. 



3dly. To go one step further: if a convex glass be so small that its 

 focus is no more tlum one-twentieth of an inch distant, we shall find 

 that eight inches, the common distance of sight, contains a hundred 

 and sixty of these twentieth parts; and, in consequence, the length 

 and breadth of an object, when seen through such lens, will each be 

 magnified a himdred and sixty times, which multiplied by a hun- 

 drc<l and sixty to give the square, will amount to twenty-five thousand 

 six hundred : and so many times, it is plain, the superficies of the ob- 

 ject nuist appear larger than it does to the naked eye at the distance 

 of eight inches. 



Therefore, in a single microscope, to learn the magnifying power 

 of any glass, no more is necessary than to bring it to its true focus, 

 the exact place of which will be known by an object's appearing per- 

 fectly distinct and sharp when placed there. Then, with a pair of small 

 compasses, measure, as nearly as you can, the distance from the centre 

 af the glass to the object you were viewing, and by afterwards apply- 

 iosi the compasses to any ruler with a diagonal scale of the parts of an 

 inch marked on it, you will easily find how many parts of an inch the 



