This one was a tough one. Paraphrasing the author: The thesis of the book is that a distinction can be made between two types of objects: one (objects of natural science) can sustain magnification; the other (objects typical of humanistic inquiry) cannot. This means that humanistic inquiry cannot match scientific inquiry. Because the objects of natural science permit magnification, we can dissect them into smaller and smaller pieces, each time discovering new, important information about them. Cultural objects, such as a photograph, will not yield to microscopic study - we generate noise, not signal.
I will admit it is kind of an odd image but I thought it made for an intriguing cover.