Since a printing press can only print "black or white" not shades of grey, the image is composed of dots to fool the eye. This is called a halftone and was perfected in 1886.
The image that you see has been etched by acid into the copper. You can clean the plate with soap and water with no fear of damaging the image as long as you do not use abrasives or inedvertantly scratch it.
From the link below. This site is about the printing of baseball items but gives and excellent summary of the history of printing techniques from engraved wood blocks to photoengraved plates like the one you found.
>>>>Halftone Printing: realistic pictures
The following discussion of the halftone printing process is jumping ahead, as it was not widely used until the late 1880s.
As the early prints used lines to make images they could not be detailed enough to look realistic. The invention of the halftone process replaced lines with dots. This allowed for much more detail. While halftone can't produce the quality and detail of a real photograph, it can make a realistic representation. This process is used today to make pictures including for newspapers, magazines and books. If you look closely at a picture in today's image is made up of tiny dots. The smaller and closer together the dots, the more detailed the image. This process could be applied to both the relief printing and lithography. Halftone applied relief printing is called photoengraving. Photoengraving is used to make images in newspapers. Halftone applied to lithography is called photolithography. This is used to make color art posters and baseball cards such as Goudeys and Topps. Your printer at home or the Xerox at work uses the halftone process to make images.
In the mid-1880s into the early 1890's newspapers replaced woodcuts with half-tone images. By the mid 1880's some tobacco cards used halftone lithography. As the technology was still new, the images were not of the quality of today's. Pictures in 19th century newspapers were coarse in detail. Early halftone lithography, in particular, was not realistic. This lithography couldn't create detailed dots. It was also more readily applied to artistic images where realism was not the prime intent. However, any print will be revealed to be halftone by its dots.<<<