324 DIRECTIONS FOR THE MICROSCOPE HISTORY, 
gives tiiis 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 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 
eye. But as the object is magnified 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 find that its superficies is 
indeed magnified sixty-four times. 
2dly. 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 lie 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 stop further : if a convex glass be so small that its 
focus is no more than 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 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 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 
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 ot an 
inch marked on it, you will easily find how many parts of an inch the 
