Week 12: Lenses and Mirrors 413
fo foαysfey’βls’Figure 177: A “Galilean” microscope uses adiverginglens for the eyepiece. This does not affect
the formula for the magnification, but it ensures that the eye seesthe tiny objectserectinstead of
inverted. As always, we use a “central” ray for the second lens that is deflected at the plane of the
first lensas ifit passes through both lenses to find the location and size of the final image.
adequateto observe e.g. blood cells, bacteria, the cellular structure of plantan animal
tissue, amoeba, paramecium, and a host of microorganisms and cellular structures. For
example, amoeba can range in size from 10-1000μm (where the latter, note well, is
roughly a millimeter and barely visible to the naked eye). A 250 power microscope can
make an amoeba appear to the eye as large as a 25 cm object, clearlyrevealing its nucleus
and vacuoles. Even small amoeba or bacteria will appear several millimeters in size at
this magnification.
Just as the telescope caused a revolution in our vision of cosmology and the structure of
the Universe at large distances and over long times, the microscopecaused a revolution
in our vision of the world of biology. Disease, which had long been thought of as being
caused by demons or by a curse afflicted on sinners by God, was seento be caused by
living organisms too small to be seen by the naked eye. Where beforethe only possible
cure for most diseases was believed to be divine intervention, miracles brought about
by repentance and prayer, the microscope enabled the discoveryof antiseptic medicine- that heat, soap and water, alcohol, and eventually antibiotics kill off disease-causing
microorganisms to prevent or cure disease quite independent of “magic” such as miracles
or prayer. The two together brought about the Enlightenment, atime of intense discovery
and invention that ultimately ushered in the rational modern world oftoday.