Two collections that were sold during the same week earlier this year here at Sotheby's were reminiscent of the ''compare and contrast'' examination questions one used to struggle with at school.
At first there appeared very little in common between the Thomas F. Flannery Jr. collection of medieval and renaissance European works of art and the W. J. Shepherd collection of ''treen'' (an old English word for small domestic items made of wood). There were great disparities in social background, age, and monetary value between the collections. Yet they were both small and portable, and they showed an equal weight of scholarship, scope, and sheer visual pleasure.
Both collections were works of art in their own right. They had stature solidly based on years of study, search, and research. They presented to the modern viewer an immensely valuable insight into the tastes and customs of the past, as well as the values lost or inappropriate to our age.
First came the Flannery collection, which winged its way to London at the express wish of the late Chicago collector. It was in every sense a ''private'' collection, previously known chiefly eamong scholars of the renaissance and medieval periods, to whom Mr. Flannery had always given generous access.
Mr. Flannery spent over 30 years patiently assembling the collection; indeed, many of the footnotes in the catalog compared items with those in great public collections in both Europe and America. Most of the early works - ecclesiastical figures and vessels - probably indicate the power of the church at this period to command the finest of the available craftsmanship. There were madonnas, saints, angels, and monks which, fashioned in ivory, gilt bronze, wood, and stone, express that quiet and refined piety so evident in the iconography of medieval Christianity. At the same time, the chalices, caskets, reliquaries, and monstrances displayed a profusion of jeweled and enameled decoration somewhat at odds with the solemnity of the figures.