What is the characteristic feature of Plasmodium vivax observed under a microscope?

The characteristic feature of Plasmodium vivax observed under a microscope is Schuffner's dots. These are small, round, reddish-purple granules found within the infected red blood cells during the examination of blood smears. Schuffner's dots are indicative of P. vivax infections and are used to distinguish it from other species of Plasmodium.

In the context of malaria diagnosis, both thick and thin blood smears are utilized, but the primary distinguishing feature for P. vivax in a thin smear is the presence of Schuffner's dots. Thick smears, while useful for detecting the presence of parasites, do not allow for the identification of specific types, including P. vivax. James' dots are not a recognized feature associated with P. vivax and are not widely noted in parasitology.