what was the purpose of stoker keeping Dracula from appearing in human form through most of the story?

Dracula is an epistolary novel; this form allows Stoker to juxtapose the rational world of the English Victorian observer with the supernatural world of Count Dracula. English men and women of Stoker's time had a strong tradition of observation and letter writing; educated English people used journals and letters to set down artful and detailed observations of their world and lives. The nineteenth century was also a time in England that glorified the blossoming of science and reason. It was the century of Charles Darwin and a period of increasing industrialization and urbanization. London was the greatest and most modern city in the world, and England, in part through science and technology, had conquered much of the world. One of the novel's themes is the clash between the world of the supernatural and unknown with the scientific and rational world of Victorian England. Jonathan Harker is the model of a modern English businessman. His journal entries provide detailed descriptions of peasants he sees and dishes he eats. He notes the quaint superstitions of the Eastern Europeans, and subsumes all he observes to a framework of science and reason. Although he has ominous dreams at the hotel, he blames the nightmares on his dinner of the evening before. When the coachman's body seems to become translucent, Harker blames the phenomenon on a trick of the eyes. His description of the coachman‹incredibly strong, and with eyes that at times seem to glow red‹is without comment. It is as if Jonathan is too modern and "rational" to recognize what the reader realizes very quickly‹even when the wolves seem to obey the command of the coachman, Harker does not remark on the event beyond saying that it happened. He becomes uneasy and fearful, but he lacks the framework to reach conclusions about what he is seeing.