3^4 DIRECTIONS FOR THE MICROSCOPEr— IIISTORY. 



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

 was ambassador in England, in 1619, Cornelius Drehell 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: fur 

 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 not be 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 

 eve. But as the object is magnified every way equally, in length as 

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

 much it api)ears enlarged, and we shall then find that its superficies is 

 indeed magnified sixty-four times. 



'2d\y. 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 furdier: if a convex glass be so small that its 

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

 that ei"-ht inches, the common distance of sight, contains a hundred 

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

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

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

 dred 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 must 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 ])!ace of which will l)e 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 

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

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

 inch marked on it, you will easily find how many part:^ of an inch the 



