Constant didn't number them, the numbers are just to distinguish them on the website.
Work
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photo: Tom Haartsen
Nébulose Mécanique liggend
1958
35.0cm x 96.0cm x 73.0cm
brass, plexiglass
Collection Fondation Constant longterm loan to Stedelijk Museum Schiedam, NL

photo: Tom Haartsen
Nébulose mécanique [III]
1958
65.0cm x 53.0cm x 43.0cm
metal, plexiglass
Collection Kröller-Müller, Otterlo, NL donated by Ida and Piet Sanders, Schiedam
photo: Tom Haartsen
Nébulose mécanique [II]
1958
136.0cm x 104.0cm x 25.0cm
aluminum, iron, marble
Collection Kunstmuseum Den Haag, NL
Constant made several Nébulose mécanique, the did not number them. The numbering is just in the title here is for practical reasons to distinguish them on the website.

photo: Tom Haartsen
Nébulose mécanique [IV]
1958
73.3cm x 58.5cm x 21.0cm
aluminum, brass, paint, plexiglass, wood
Collection Daniel and Estrellita Brodsky, USA

photo: Tom Haartsen
Nébulose Mécanique [V]
1958
36.0cm x 15.0cm x 43.0cm
aluminum, plexiglass, steel
Private collection, Belgium
photo: Tom Haartsen
Ruimtecircus [II]
1958
105.0cm x 90.0cm x 100.0cm
copper, iron
Collection Kunstmuseum Den Haag, NL
1958-1959

photo: Tom Haartsen
Construction dans un volume
1957
142.0cm x 49.5cm x 21.5cm
iron, paint
Collection Fondation Constant longterm loan to Stedelijk Museum Schiedam, NL

photo: Tom Haartsen
Eivormige constructie [II]
1957
103.0cm x 63.8cm x 60.6cm
plexiglass, steel, stone
Collection Fondation Constant longterm loan to Cobra Museum of Modern Art Amstelveen, NL

photo: Tom Haartsen
Eivormige constructie [I]
1957
59.0cm x 48.2cm x 53.0cm
metal wire, plexiglass
Collection Rijksmuseum Twenthe, Enschede, NL

photo: Tom Haartsen
Constructie geel en wit
1956
83.5cm x 78.4cm x 71.5cm
aluminum, plexiglass
Collection Rijksdienst voor Cultureel Erfgoed longterm loan Stedelijk Museum Amsterdam, NL