164
8 In the Meantime in Groningen
for very long; he obtained a Ph.D. in chemistry in 1915 (at the University of
Amsterdam) and was subsequently appointed lecturer in theoretical physics
at Groningen. His position at the Astronomical Laboratory was taken by
Kapteyn’s later successor, Pieter Johannes van Rhijn.
Others who worked on a dissertation with Kapteyn did so while not being
employed as his assistant, but either at their own expense or on temporary
appointments. Lambertus Yntema was Kapteyn’s third Ph.D. student. He was
born in 1879 in Friesland and obtained his Ph.D. degree in 1909. After this he
became a teacher and director of secondary schools in Breda, Leeuwarden and
Amsterdam. His dissertation concerned the total amount of starlight reaching
us from the sky. In presenting his Plan of Selected Areas, Kapteyn had identified
this as an important constraint in testing models for starlight distribution in
space. After all, each model makes a prediction of the total number of faint stars,
even for the ones that cannot be detected individually, even with a telescope.
One can add up the light of all stars including the faintest, and that gives a
value for the brightness of the background sky. And that must be in accordance
with the observed value.
Measuring the brightness of the night sky—at a dark site, of course, and
without the Moon being above the horizon—is no easy task. Kapteyn had
experimented with it when he was in South Africa by holding up a sheet of white
paper and illuminating it from behind him so that it was brighter than the sky,
then turning the lighting down until it no longer contrasted with the sky and
became a starless spot otherwise indistinguishable from the background sky.
Together with Kapteyn, and with the help of the physics laboratory’s workshop,
Yntema designed two special setups to make an attempt (see Figs. 8.6 and 8.7).
The way the first device worked was similar to Kapteyn’s earlier experiments.
In Fig. 8.6 we see a long, thin pole, which was pointed towards the sky and
along which a lamp could move. In the figure it is just above its center of
the pole, indicated with the letter ‘M’. At the top left end was a small screen,
illuminated by the lamp (‘Q’). Just below it was a circular plate with a hole in
it on a bracket placed slightly next to the pole (‘S’), allowing the observer to
look at the screen. In the screen ‘Q’ was also a hole, through which the night
sky could be seen and compared to the brightness of the surface of the screen.
To prevent scattered light, the part with screen ‘Q’ and picture ‘S’ was placed
in a box that is seen in the picture to the left side on a stand. This box (‘T’)
had two holes, the larger one to let the light of the lamp through, the smaller
one for the observer to look through.
The measurements were carried out by Yntema in Borger, a village 36 km
south-east of Groningen. His wife made all the notes. They were sitting on
a flat roof, surrounded by a blinding fence. They checked the stability of the
8 In the Meantime in Groningen
for very long; he obtained a Ph.D. in chemistry in 1915 (at the University of
Amsterdam) and was subsequently appointed lecturer in theoretical physics
at Groningen. His position at the Astronomical Laboratory was taken by
Kapteyn’s later successor, Pieter Johannes van Rhijn.
Others who worked on a dissertation with Kapteyn did so while not being
employed as his assistant, but either at their own expense or on temporary
appointments. Lambertus Yntema was Kapteyn’s third Ph.D. student. He was
born in 1879 in Friesland and obtained his Ph.D. degree in 1909. After this he
became a teacher and director of secondary schools in Breda, Leeuwarden and
Amsterdam. His dissertation concerned the total amount of starlight reaching
us from the sky. In presenting his Plan of Selected Areas, Kapteyn had identified
this as an important constraint in testing models for starlight distribution in
space. After all, each model makes a prediction of the total number of faint stars,
even for the ones that cannot be detected individually, even with a telescope.
One can add up the light of all stars including the faintest, and that gives a
value for the brightness of the background sky. And that must be in accordance
with the observed value.
Measuring the brightness of the night sky—at a dark site, of course, and
without the Moon being above the horizon—is no easy task. Kapteyn had
experimented with it when he was in South Africa by holding up a sheet of white
paper and illuminating it from behind him so that it was brighter than the sky,
then turning the lighting down until it no longer contrasted with the sky and
became a starless spot otherwise indistinguishable from the background sky.
Together with Kapteyn, and with the help of the physics laboratory’s workshop,
Yntema designed two special setups to make an attempt (see Figs. 8.6 and 8.7).
The way the first device worked was similar to Kapteyn’s earlier experiments.
In Fig. 8.6 we see a long, thin pole, which was pointed towards the sky and
along which a lamp could move. In the figure it is just above its center of
the pole, indicated with the letter ‘M’. At the top left end was a small screen,
illuminated by the lamp (‘Q’). Just below it was a circular plate with a hole in
it on a bracket placed slightly next to the pole (‘S’), allowing the observer to
look at the screen. In the screen ‘Q’ was also a hole, through which the night
sky could be seen and compared to the brightness of the surface of the screen.
To prevent scattered light, the part with screen ‘Q’ and picture ‘S’ was placed
in a box that is seen in the picture to the left side on a stand. This box (‘T’)
had two holes, the larger one to let the light of the lamp through, the smaller
one for the observer to look through.
The measurements were carried out by Yntema in Borger, a village 36 km
south-east of Groningen. His wife made all the notes. They were sitting on
a flat roof, surrounded by a blinding fence. They checked the stability of the