Authenticity is the core of any museum's mission. We take it for granted that when we stroll through any reputable art museum we're being shown real Manets and Monets, not copies or fakes.

Likewise, we assume that science and history museums are displaying authentic specimens, instruments, documents, and artifacts, not imaginary or simulated ones.

Perhaps you've noticed that science museums, eager to broaden their appeal and boost income, have begun to adulterate this paradigm. Through Sept. 20, the Franklin Institute is beating the promotional drums for a Star Trek show, a slick amalgam of fantasy, pseudo-science, and popular entertainment.

I was halfway through "Fashion in Film" at the Allentown Art Museum when I began to wonder if the same trend was beginning to infect art institutions. I realized that I wasn't sure what I was looking at - not in the usual sense of not comprehending the material but in the sense that I couldn't determine if the garments were historically authentic.

"Fashion in Film," a traveling show organized by the Trust for Museum Exhibitions in Washington, presents 36 movie costumes, most of them gowns but also including a few ensembles for men. Almost all were created by a London firm called Cosprop, which specializes in film, television, and theater costumes for the 18th, 19th and early 20th centuries.

The exhibition is conceived for popular appeal. The costumes are identified with the films for which they were created - Gosford Park, Sense and Sensibility, Dangerous Liaisons - and with the celebrity actresses who wore them, from Elizabeth Taylor and Cate Blanchett to Anjelica Huston and Madonna.

No problem there - no one is pretending that film costumes are great art. And a regional museum such as Allentown needs exhibitions like this one in its mix if it's to remain viable.

No, the problem is that the show's subtitle is "Period Costumes for the Screen," which prompted me to wonder what "period" meant in this context. Were the clothes on display absolutely authentic to their periods as either historical artifacts or replicas of such? Or were they contemporary interpretations of what women of means could have been wearing in the 18th and 19th centuries?

The answer, supplied by the museum's textiles curator, Jacqueline M. Atkins, is all of the above. According to Atkins, most of the clothes are modern interpretations based on designs from the periods depicted. Many gowns represent the Regency period of the early 19th century, the high Victorian era, and the Edwardian years of the early 20th century.

"In many cases," Atkins explained, "the designers tried to use fabrics of the time and not to use synthetics. They tried very hard to replicate the type of fashions being worn."

To this end, the costumers sometimes incorporated bits of authentic period materials into their gowns. For instance, the collar piece on a wedding gown worn in Sense and Sensibility dates from 1810. Two-thirds of the lace on a gown worn by Lee Remick in The Europeans is authentic to its period.

Two outfits in the show are exact replicas, "down to the buttons," of historical garments. And two other gowns are entirely authentic - a 1912 red number worn in Titanic and a slinky white sheath from the 1930s used in Gosford Park.

To further enliven the mix, Atkins has interposed some samples of historical textiles from the museum's extensive collection. Knowing that these exhibits were authentic and that most of the dresses were not, I found the simple samples more satisfying aesthetically than the far more ornate concoctions produced by the costumers.

For a costume show, does such hair-splitting matter? Not if viewers understand that "Fashion in Film" is about the craft of costume design. This is key to enjoying a "historical" exhibition in which much of the material is relatively new.

As Atkins emphasized, viewers must approach "Fashion in Film" with the understanding that these gowns present superior, sometimes spectacular, examples of the costumer's craft. Taken on those terms, this handsome and thoroughly enjoyable show makes sense, even to me, for whom fashion design is as impenetrable as Basque.

Authenticity is never an issue with "Waves, Waterfalls and Ripples: Water in Japanese Art," a small show that the Allentown museum organized from its own collection. In terms of both how the concept is expressed and the quality of the objects chosen to do so, it's as close to perfection as one could expect.

The main body of the show consists of 17 Japanese woodblock prints from the 19th century, most of them by the two supreme masters of this medium, Katsushika Hokusai and Ando Hiroshige.

Six prints represent a suite by Hokusai called A Journey to the Waterfalls of All the Provinces. In these, the artist examines the myriad forms that falling water can take, from a long, threadlike arc to a highly ramified cascade that resembles the roots of a banyan tree.

Hokusai's images aren't conventionally scenic; they're slightly abstracted and executed in a few muted colors, a style that reminded me of Cezanne's landscapes.

Hiroshige's treatment of water is more picturesque, and often focuses on the sea as an element in a panoramic composition. Several of his prints are from the famous series Thirty-Six Views of Mount Fuji. In contrast to Hokusai's more subtle palette, Hiroshige favored bold primaries such as red and blue combined with black and white.

Sumptuous as they are, the prints are dominated by a truly spectacular textile, a four-panel folding screen in which naturalistic waves are delineated in silk embroidery. Where the prints represent a pure Japanese aesthetic, the screen image is emphatically naturalistic, and Western.

The four embroidered panels, on silk backing, were designed and executed by a man, Hashio Kiyoshi, also known as Seizaburo Kajimoto, as a commission for the Japanese government for exhibition at the 1915 Panama Pacific Exposition in San Francisco.

Set into a black lacquered frame with gilded mounts, Morning Sea won the Medal of Honor for its spectacular craftsmanship and powerful depiction of agitated waves. Viewers can't get close enough to determine the fineness of the stitching, but the shimmering color shifts in the waves are impressive even from a distance.

The screen recently came to the museum as a gift from Mr. and Mrs. Van Santvoord of New York. Mr. Santvoord is a descendant of Kate Fowler Merle-Smith, a collector who made extensive donations of textiles to the museum.

Art: Fashion in Film

"Fashion in Film" continues at the Allentown Art Museum, 31 N. Fifth St., through Aug. 9. "Waves, Waterfalls and Ripples" continues through July 18. Hours are 11 a.m. to 5 p.m. Wednesdays through Saturdays and noon to 5 Sundays. Admission is $6 general, $4 for seniors and students, and $3 for visitors 6 through 12. Free Sundays. There is a $5 surcharge for "Fashion in Film" at all times. Information: 610-432-4333 or www.allentownartmuseum.org.

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Contact contributing art critic Edward J. Sozanski at esozanski@phillynews.com or 215-854-5595. Read his recent work at http://go.philly.com/edwardsozanski.