Time machine.

Hubert-Jan Henket's additions to the distinguished nineteenth-century Teylers Museum in Haarlem - Holland's oldest museum - celebrate Modernism's regard for transparency and spatial interpenetration.

Clarity of form is not always synonymous with clarity of thought: a point missed by some of the unsuccessful entrants in the competition won by Hubert-Jan Henket, back in 1990, for the extension of Holland's oldest museum. Although the Teylers cannot compete with the size and popular appeal of the great museums of Amsterdam or Rotterdam, its rich history and intense, introverted galleries, largely hidden within a central Haarlem city block, make it a unique institution. The Teylers' incremental development, in curatorial as well as architectural terms, has placed content ahead of form. Hubert-Jan Henket's intervention continues this pattern.

Since its foundation in 1778, recognising the philanthropic concerns of the late Pieter Teyler van der Hulst, a wealthy textile merchant, the museum has pursued a mission to educate Haarlem's less advantaged citizens in a broad range of disciplines. Teyler's own collection of instruments, books and fossils was soon supplemented by 1500 drawings, including works by Michelangelo. Raphael and Rembrandt, purchased from the estate of Princess Christina of Sweden in 1790. The Teylers' scientific exhibits were equally impressive. Numerous internationally eminent researchers have worked within the museum's intense interiors, focused on the toplit Oval Room, which stands at the heart of the virtually landlocked site.

To accommodate the growing collection, the Teylers was extended in the 1870s, along an axis leading from the Oval Room towards a new entrance facing the river Spaarne. The nineteenth-century galleries - designed by the Viennese architect Christian Ulbrich - sufficed until 1990, when the museum identified a number of additional facilities it required to meet the expectations of contemporary visitors and curators. These included a temporary exhibition gallery, a laboratory suitable for public demonstrations, a restaurant, and specialised areas for the storage of drawings and rare books. A competition was held, with the brief to fit these functions within the existing museum, an adjacent redundant 1950s printworks, and such new accommodation as might be required to improve the circulation throughout the complex.

Hubert-Jan Henket's winning scheme - selected from a field of 169 - stood out not so much for its conceptual boldness or formal elegance, but for its sensitivity to the quality of the existing buildings and their exhibits. Henket's extension to the Boymans-Van Beuningen Museum in Rotterdam (AR July 1991) set a steel and glass pavilion distinctly apart from its masonry host building; while his more recent law courts at Middelburg (AR July 1996) demonstrated an interest in a variety of contextual issues, mostly addressed with abstract, compositional devices. Measuring about 1000 sqm, the Teylers design, while smaller than either of these forerunners, embodies an altogether more complex synthesis of technical and contextual concerns.

Henket describes the Teylers as 'a perfect and beautiful time machine, where the atmospheres of various periods are still intact'. The Oval Room appears today precisely as depicted in Wybrand Hendriks' oil painting of circa 1810. The architect's analysis of the museum emphasised the varied qualities of daylight, the density of exhibits, and the galleries' introspection. Nowhere in the nineteenth-century building was the visitor permitted a glance away from the exhibits, to the outside world. Henket's prime concern became the retention of these unique qualities, avoiding compromise to the existing daylighting.

By contrast, Henket's additions celebrate Modernism's regard for transparency and spatial interpenetration. The museum garden has been rediscovered, new vistas opening the depths of the building to its mature trees. The garden's open area having been reduced, the necessarily blank facade of the temporary exhibitions gallery is clad in grey glass, giving a play of reflections and shadows which creates an illusion of greater space and transparency. To avoid excessive contrasts in lighting levels, Henket's inside-outside spaces are provided with generously overhanging roofs, and surfaces opposite large expanses of glazing are rendered in darker colours.

Always respecting the museum's intimate scale, and subject to planning and arboricultural controls, Henket's scheme is intended to ease movement and facilitate visitors' orientation throughout the building. The competition-winning scheme consists of several distinct elements, the largest of which is a temporary exhibition gallery, orientated parallel to the old building's principal axis, but separated from the nineteenth-century structure by a small garden. Henket's other major extension is grafted onto the flank of an existing building, and establishes a cross axis from the Oval Room towards a new secondary entrance and restaurant. The new entrance foyer also provides a link to workshops and stores, located within the refurbished Zegelwaarden printworks. By opening up the roof over the new cross axis, Henket allowed views from the gallery interiors towards the seventeenth-century Bakkenesse tower in one direction, and the eighteenth-century astronomical observatory - a powerful icon of the Enlightenment, perched above the Oval Room - in the other.

Free of the inhibiting neutrality expected of exhibition galleries, the secondary entrance area, overlooked by the mezzanine restaurant, is the most spatially dynamic part of Henket's design. A fully glazed corner is flanked by dislocated, overlapping vertical and horizontal planes, calling to mind some of the more flamboyant examples of Dutch architecture of the 1920s. All of the elements introduced in Henket's project are united by their orthogonal geometry and detailing, both of which reinterpret characteristics of the pre-existing buildings. The tactile nineteenth-century exhibits, and the warmth of the timber-panelled galleries, suggested two of the new building's most distinctive details: elegantly profiled laminated timber beams, and veneered wood mullions.

The temporary exhibition gallery is necessarily the most neutral of the museum's spaces, blending natural and artificial lighting in a space whose characteristic twentieth-century horizontality contrasts with the more classical proportions of the earlier interiors. Despite the building's apparent simplicity, the structural and environmental strategies are inventive and fully integrated in the design. Laminated timber beams support a roof section incorporating mechanical services and electrically operated louvres, controlling daylight already tempered by an outer canopy of printed glass. Much of the timber is only exposed externally, with the northern wing's beams supporting a plywood-faced soffit, hovering over a boarded deck. The partial concealment of beautiful timber sections does not compromise the reasons behind the material's selection, which are as much to do with structural efficiency and ecological sustainability as with aesthetics.

Hubert-Jan Henket's extension to the Teylers has brought the museum's facilities up to date, and created an enjoyable and coherent sequence of spaces, without sacrificing the unique character of this 'time machine' to the commercialism and museological gimmickry which has turned many larger institutions into places of light entertainment, at the expense of those more seriously concerned with research and public education. The restraint of Henket's architecture is such that, with the continued good stewardship of the curators, the additions of the late twentieth century might become as timeless and responsive to the museum's future needs as those of earlier periods.