The gentleman looked a little curiously at the ragged figure before him.
A theme emphasized in Ragged Dick is that of appearance: appearances as a sign of rank or wealth or how appearances can be incredibly deceitful, for example. Even in the short passage above, this theme is protrayed. Obviously, there are many assumptions made based solely on how someone looks. Thus, Frank was hesitant in wanting to walk around New York with Dick, who is "so ragged and dirty." Frank, who apparently is the son of a gentleman with some stature, does not want to be seen with Ragged Dick, a lowly boot black. Their contrasting appearances might be offensive to Frank. Mr. Whitney also bases his opinion of Dick on how Dick appears. He observes that Dick "looks honest. He has an open face." He therefore concludes that, despite societal conventions, there was something in Dick's face that made Mr. Whitney think that Dick could "be depended upon." Mr. Whitney was therefore not hesitant to trust his nephew in the hands of Ragged Dick.
The "servant of the hotel" also judges Dick by the way he loks. The attendant "stopped Dick" until Mr. Whitney, a wealthy man, "explained that he has something for [Dick] to do." Dick's appearance suggested that he was poor, homeless, desperate, and therefore, not allowed in the posh hotel. People assume that he cannot be trusted, simply because he's dressed poorly.
Another pattern that arises in this passage is the use of the word "know." Throughout the novel, Ragged Dick calls himself "ignorant." He especially declares this fact to Frank, as well as his desire to "grow up respectable." However, during the time that we are introduced to Frank -- during the tour around the city -- it is Dick that seems much more knowledgable. In this passage, Mr. Whitney asks Dick if he knows all the public buildings and the Central Park. Dick responds, "I know my way all round." The next line down, Mr. Whitney is said to look "thoughtful" and yet, in the next line, he replies, "I don't know." I believe Horatio Alger is trying to drive home the contrast that despite all that their position and wealth would imply -- Mr. Whitney and Frank being respected, wealthy gentlemen, and Dick being a homeless book black -- Dick is much more knowledgable than he would first appear.