Trade cards were a means of advertising products or services and thereby attracting customers to the owner's shop. They often included a variety of details about the proprietor and his business, and illustrated his wares. Cards for the scientific instrument industry depicted all classes of instrument and the products from which they were made. A careful study of the cards can reveal much supplementary information about the way the industry worked, so their use, and limitations, as a source of historical (...) evidence is discussed in detail. Over sixty English trade cards from London and the provinces, covering the period 1670 to 1900, are illustrated . By careful analysis, it is shown that aesthetic style and type-face design, combined with written and pictorial content, may be used to deduce dates and business practices. Many small pieces of information gradually build a complex picture of an important and fascinating industry. (shrink)