Engravers would etch, by hand, an image onto a copper or steel plate. This would then be coated with ink, the excess wiped off, leaving ink in the groves made by the engraver, and then pressed onto a sheet of paper. The paper would draw the ink out of the groves, and a black and white image would result.
If one wished a different sized image, or if the plate simply became worn, one would have to re-engrave a fresh plate.
Artists like Loggan could create shades of grey by using lines whose spacing could be varied, and which could become broken. Not until the nineteenth century did it become possible to create shades of grey using variable-sized dots, as are also used in modern printing.
Below is a reproduction of Loggan's print of St John's College.
In front, and to the left, of the main gate is a dog, half in the shadows. It is rather difficult to spot, but one can click on the above image for a larger view into to see the detail better.
To the left is a 1,200 dpi scan of the dog from the image which appears in The Architectural History of the University of Cambridge (1886). Whilst this is not from Loggan's orginal plate, but a reproduction, the technique is the same as Loggan would have used, with horizontal line producing shading, and breaks in the lines lighening the shade.
The width of the lines is around three pixels, and their separation around eight pixels, so there are about 150 lines per inch, each of which has a width of about one four-hundredth of an inch.
To the left is a 1,200 dpi scan of the same dog from the image which appears in Cantabrigia Illustrata (1905). Whilst this claims to be a reproduction of Loggan's earlier work, the actual execution is very different. Now one sees that the shadows are made up from varying sizes of dots, not of horizontal lines.
The dots form a regular grid, at 45 degrees to the horizontal, and with a spacing of around 150 dpi. Several different sizes of dot are used. The precise technique used by this volume is not stated.
The natural size of the two images above is about two fifths of an inch (or one centimetre). One would be hard pressed to tell them apart by eye, and, if one can spot any difference, the shading in the newer image might appear superior.
The whole image is about 7¾"x9¼", with a centre fold, in The Architectural History of the University of Cambridge, and about 9"x10½" with no fold in the 1905 version of Cantabrigia Illustrata. The version in The Architectural History lacks the arms of, and dedication to, Dr Peter Gunning, Master of St John's from 1661 to 1670, and the key is moved to the bottom of the print.The Cantabrigia Illustrata version has eight lines of Latin text at the bottom, which have a total height of half an inch, and so are almost illegible. I believe that the image in the 1690 version of Cantabrigia Illustrata was significantly larger, about 16½" wide rather than the 9" wide of the 1906 reprinting. In contrast, van der Aa's reproduction from 1707 is about 5" wide, lacks a few boats on the Cam (it has two in a row at the right, not five), has fewer people in the street in front of the College, places the arms of the College in the centre below the image, in the middle of the text "Le College de S. Jean", and omits the dedication to Gunning and the Latin description of the College.
Back to Cambridge prints.