FOR MONITORING CHANGES in manuscripts, scholars and conservators have long recognized the value of photography. In the 1980s, to check for damage, conservators at the British Library compared earlier photographs of the Lindisfarne Gospels to the manuscript. However, assessing cultural heritage through photographs has its origins in the earliest days of photography. For example, shortly after Louis-Jacques-Mandé Daguerre successfully demonstrated his daguerreotype in Paris (1839), the French Commission des Monuments Historiques hired five photographers to capture images of the nation's architectural heritage. Responsible for its preservation, the Commission wished to assess the conditions of historic sites throughout France, many not accessible by rail. This lack of access by rail echoes access to the past conditions of manuscripts. Both require photography. Now, the digital opens ways for understanding these past conditions and the aging processes of manuscripts by extending what can be known from historical photographs.

Digitized photographs provide three major advantages over printed ones. First, once digitized, photographs can be overlaid, and the transparency of the top image adjusted. This eliminates reliance on side-by-side comparisons and looking back-andforth, making losses easier to identify. Second, while overlay benefits identifying losses, it becomes more beneficial when images align. Because the content of digitized images is malleable, such alignment is possible, a digital process called registration. Through overlay and transparency, comparing registered images makes smaller differences more apparent. It also enables identifying differences through computer-assisted means, such as subtracting one image from another. Through subtraction, resulting images reveal miniscule changes that otherwise likely go unnoticed, such as losses measuring .1 mm in diameter. Third, digitizing historical photographs preserves them in a digital form, allowing them to be collected and redundantly stored, safeguarding their irreplaceable content.

In this chapter, I demonstrate the value of and provide an approach for digitizing, registering, and comparing historical photographs. I make this case, however, not simply to propose a practice worthy of becoming best practice whenever digitizing a manuscript. Because of its unprecedented benefits, I want to encourage its practice even when a major digitization of a manuscript is not at hand. Information gained from digitizing historical photographs is far too important to ignore and too valuable to risk losing. In my work with the St.