Nevertheless there are limits: e.g. a "Coddington" type
magnifier (like the one shown in our previous issue) with meager three millimeters working distance
and a recessed lens mount will already range beyond the limits of our direct sunlight method.
But be consoled - we are not alone with those limits. The following scientific
anecdote might help to illuminate the risks of measuring any of those subtle
optical properties: the so-called "Utrecht lens" microscope, made by the famous
amateur Antoni van Leeuwenhoek centuries ago, is housing an extremely tiny lens (1.3 mm in diameter).
Due to understandable handling restrictions it was strictly forbidden to dismantle
the precious instrument in order to investigate this lens directly. Only non-destructive tests (NDT)
were permitted. As a consequence this instrument marked a particular challenge
for optical analysts.
A reknowned Dutch specialist, Jan van Zuylen (see link below), had developped
a dedicated and sophisticated method in order to measure the optical properties
of the Utrecht lens. In 1981 he published his results with the conclusion
that the Utrecht lens was aspherical, probably produced by a complicated glass-blowing variant.
This finding contributed to the enormous fame of Antoni van Leeuwenhoek,
adding a first "aspheric" to his long list of achievements.
But as time and technoloy went on, a modern NDT method came up:
the so-called neutron tomography was able to prove by means of direct image
that the Utrecht lens is in fact a rather primitive, normal though tiny
globular glass sphere. This interpretation was corroborated by the fact, that a small
glass stem fragment became visible on one side of the sphere. This stem can easily be explained
as the typical trace of the most easy-going micro glass lens production method:
melting the end of a slim glass stick in a flame until it is forming a glass sphere.
This method is described in the respective and impressive experimental work
by a German Leeuwenhoek specialist and microscope collector [Klaus Meyer 1991].
|