PERFORMING ITS function with verve in its brief decade of existence, the National Portrait Gallery has introduced us to our past through a series of vivid, imaginative special exhibits of which the latest has deservedly achieved permanence in this volume. The daguerreotype-when successful, which was not always-was a device for portraiture of extraordinary penetration, capable of capturing and holding the actuality of a living person as if the blood were still warm. In these assembled portraits it is as if a door were flung open on a hall filled with the most significant and memorable figures of mid-19th century America as they appeared in the two decades before the Civil War, from seared old Andrew Jackson at the end of his life to a prodigiously self-assured Samuel Clemens at age 12.

This first functioning photography, developed by Louis Jacques Mande Daguerre and Joseph Nicephore Niepce in the 1830s, and improved in the United States by Samuel F. B. Morse to reduce the exposure time from minutes to seconds, thus making portraiture feasible, reached its summit in this country. Under sunlight, the sitter's image was impressed through the camera's lens on to a chemically treated light-sensitive plate and subsequently developed and fixed by chemical process on the original plate. From pose to finished product, the procedure could be completed in 15 minutes. No negatives to be retouched and printed were involved; the concoctions of an Avedon or a Karsh were yet to come. The photographer's hand is not present in these portraits; only the subject counts. The daguerrean's philosophy was to present his sitters "as they were."

In that era of the steam engine, the reaper and the telegraph, here was one more marvel of technical progress. Suddenly everyone-not only the great or the occasional family stiffly immortalized by a portraitist in paint-could have a picture of himself, his family and friends. Daguerreotyping swept the country; by 1845 studios flourished in every city and town and itinerant practitioners traveled the roads. In the space of 15 years, millions of faces from slave to president were immortalized until, by about 1860, with the introduction of negatives and paper prints offering a more flexible and cheaper process, the daguerreotype's reign was over. "This most beautiful of all photographic techniques," in the enthused photo curator's words, was abandoned, proving Herblock's principle, "If it's good, they stop making it."

Ninety-three individuals-and their era-are here: the artists, including Rembrandt Peale and Thomas Cole; the writers, Whittier and Longfellow, Emerson and Thoreau; presidents and politicians, women, children, blacks, and a survivor of the Revolution, Albert Gallatin at 88, looking straight at us through lively eyes with a half smile that remembers all he has witnessed. Historical meaning has been added by grouping linked persons: the Beechers and Stowes; the Abolitionists; the Senate's three giants, Clay, Calhoun and Webster, the three historians, Prescott, Bancroft and Parkman; and four members, including Horatio Alger, of the Harvard class of 1852. Striking individuals find their own place. The intense face of Morse, founding father of the process, with that special handsomeness peculiar to Victorians, opens the book. The artist-engraver Asher Durand looks as alive as if he had just ceased speaking and were listening to someone's reply. The hardships he endured mark John Fremont's ravaged face. The serene loveliness of Annie Adams Fields is a feminine ideal. The rectitude of her Boston neigbor, Maria Chapman, goes oddly with a remarkable coiffure of curls on top, ringlets hanging over the ears, and a burn of intricate braids tightly would behind.

"Were you ever daguerreotyped, o immortal man?"wrote Emerson. "And did you look with all vigor at the lens of the camera . . . to give the picture the full benefit of your expanded and flashing eye? and in your zeal not to blur the image . . . did you feel every muscle becoming every moment more rigid?" And when at last relieved of his "dismal duties," did the sitter find coat, hads, shape of head all perfect, "but unhappily the total expression escaped from the face and the protrait of a mask instead of a man?"

While that result was true in some cases-Lincoln's for one, Emerson's own for another-more often the camera seemed to have a power to dissolve a curtain and discover the character behind. William Seward's hard cynicism, comes through like a blow; the fanatic glares through the eyes of John Brown; self-datisfaction sits smugly on the face of newly married Mary Todd Lincoln; in a stunning portrait the vulnerability, longing and lineliness of 17-year-old Emily Dickinson is naked to the world. The most surprising is Longfellow without his beard-a virile, unconventional face with strong sensual lips and mussed up hair, anything but what might be expected of the author of "The Children's Hour."

The most "speaking" of all is a double portrait of Rutherford Hayes and his wife Lucy taken in 1853 three months after their marriage. Side by side, both wearing the same expression of confident happiness, they are so together, so combined as to seem to want to transmit all they feel to the camera. It is beautiful protrait, not of two individuals but of a marriage, and it is not surprising, as the accompanying text tells us, that even after becoming after becoming president of the United States, Hayes considered his union with Lucy the most interesting fact of his life.

The texts are worthy of the pictures, as is the habit in the National Portrait Gallery's exhibits. A flaw in this volume is the failure to include the date or approximate data or approximate date of the portrait in the caption where it could be easily ascertained instead of leaving it to be sought, somethings in vain, through the text. Otherwise the texts redouble the pictures' value because the editors have taken pains to find material connected with the circumstances of the portrait, rather than contenting themselves with routine biographies. Our curiosity evoked by the romantic Hayeses is satisfied by an account of their courtship and by Lucy's own comments about the joint portrait. The records of Miss Lyon's Mount Holyoke Seminary attended by Emily Deckinson tell of a daguerreotype studio opened for a few weeks across the way and of "quite an excitement" among the young ladies going to sit for their portraits, if they could pay for them without stinting on missionary contributions. For every portrait, relevant quotations have been culled from letters, diaries, public records and the comments of contemporaries.

Often these tell as much about the era as the sitter. Of Lucretia Mott, the incisive, undaunted Quaker abolitionist and feminist, Emerson wrote a wonderful, if unconscious, evocation of his period. "No mob could remain a mob where she went. She brings . . . that propriety which every man loves, directly into this hurly-burly, and makes every bully ashamed." Ladies could do that in 1850. Gone now too is the impassioned eloquence of the Reverend Theodore Parker, fearless in his denunciation of the Fugitive Slave Law and of the eminent Webster for upholding it in the name of law and order and established property. Though much was amiss in 1840-60, its worth turning back to that era and its faces. CAPTION: Picture 1, Emily Dickinson; Picture 2, Frederick Douglass; Picture 3, Harriet Beecher Stowe; Picture 4, John Brown; Picture 5, Daniel Webster