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348
RECENT TIMES.
time possible, by the invention of various instruments
which took place at that time. The telescope enabled the ",
eye to see into the distance : the microscope opened out f
into view the world of small things. By these two optical %
aids the power of human vision was strengthened in an
unexpected manner and paths were opened to investiga- j
tion, which had before been beyond the boundaries of
human knowledge. The seat of these inventions was
Holland. It is doubtful to whom priority belongs in these
discoveries : but it appears that the brothers JANSSEN, who
in the beginning of the 17th century lived in Middelburg
as glass cutters, have the best claims, at least in respect of
the compound microscope.
It is not my business to go more closely in this place
into the history of this discovery and it is also unnecessary -
since it has already been discussed by HARTING in a fairly
exhaustive manner.* The extraordinary importance of the
microscope in the study of natural sciences cannot be
described in words.
The instruments were gradually improved and rendered
more complete in numberless ways. The discovery of the
reflecting telescope by JAMES GREGORY, that of the
micrometer (crossed threads) by ROBERT HOOKE, the
construction for the first time by M. HALL of achromatic
lenses by a combination of crown and flint glass, etc., were
added afterwards.
Even then men ventured to attack the difficult problems
of light and colour. The great philosopher DESCARTES
(CARTESIUS), to whom mathematics owes the recognition
of the meaning of the negative roots of equations and the
foundation of analytical geometry, sought for an explanation
of the rainbow and in this way worked out the law of incident
and reflected rays, and the angles formed by them, i
SNELL established the relations between different media
and the refraction of light in them, and GRIMALDI dis-
* P. HARTING: Das Mikroskop, ins deutsche iibers. v. THEILE, iii. TheiL
Braunschweig 1866.