Question About Book

I have just finished reading it, and have only one more qustion to address. When Watson was on top of that guy's roof (cannot remember his name now) and he was looking through the telescope, he was a boy carying food to someone. This later proved to be Cartwright.

My question is, as Watson met Cartwright in London earlier in the book, wouldn't be able to recognise him immediately after seeing him?

No one's answered it still?

I believe Cartwright was disguised as a urchin/peasant boy. I'm quite sure Holmes discloses this to Watson upon their meeting up in the hut.