Lenses and Waves

1655-1672 - DE ABERRATIONE 63
improvement of the telescope was the result of the artisanal process of trialand-error.
Better configurations were designed by making them, not made by
designing them.
3.1.2 INVENTIONS ON TELESCOPES BY HUYGENS
After Tractatus followed a decade of practical dioptrics, that was crowned by
the publication of Systema Saturnium. Together with his brother, Huygens had
become a skilled telescope maker and could already pride himself on some
innovations of the instrument. In the previous chapter, one of these
innovations has been discussed: a device to make telescopic measurements.
It is not known how Huygens discovered the principle of the micrometer.
The discovery was probably related to an innovation of the telescope he had
developed somewhat earlier: the diaphragm.
The diaphragm improved the way images were enhanced by blocking part
of the light entering the telescope. Early in 1610 Galileo discovered that
telescopic images became more distinct when he covered the objective lens
with a paper ring. 45 He determined the optimal size and shape of the ring by
means of trial-and-effort and did not try – at least not on paper – to explain
the effect dioptrically. As contrasted to such an aperture stop, a diaphragm is
inserted into the focal plane. It has the advantage of diminishing the effect
we call chromatic aberration. In December 1659 Huygens first employed a
diaphragm in his 23-foot telescope. 46 As he related in 1684:
“N.B. In 1659 in my system of Saturn, I have taught the use of placing a diaphragm, as
it is called, in the focus of the ocular lens, without which those telescopes cannot be
freed from the defects of colors.” 47
Apparently, he recognized the combining a diaphragm with some measuring
device a bit later. 48 The fact that an object inserted in the focal plane casts a
sharp shadow over things seen through the telescope seems a logical
consequence of Huygens’ understanding of the dioptrics of a Keplerian
configuration. Still, it took him some time to recognize its usefulness and this
may well have been a chance discovery. The fact that a diaphragm reduces
‘the defects of colors’ did not follow from his dioptrical theory and had to be
discovered in practice.
Until the 1660s, Huygens’ approach to telescope making did not differ
substantially from that of an ordinary craftsman. We have seen his
unmatched understanding of dioptrical theory but it cannot be told what role
it played in his practical pursuits. In Systema saturnium, he described his
micrometer in a procedural way, without explaining it analytically in
dioptrical terms. The book contained only one dioptrical passage. He wrote
45
Bedini, “The tube of long vision”, 157-159.
46
OC15, 56.
47
OC13, 826. “N.B. me anno 1659 in Systemate Saturnio meo docuisse usum diaphragmatis quod vocant,
in foco ocularis lentis ponendi, absque quo colorum vitio haec telescopia carere non poterant.” In 1694 he
explicitly claimed that he was the first to use a diaphragm: OC13, 774.
48
McKeon, “Les débuts I”, 237.