Glee Season 1 Episode 19 (Glee s01e19) with title “Dream On” airs on Tuesday, May 18th 2010 at 09:00 pm via FOX.
Here is summary for Glee Season 1 Episode 19 :

Mr. Schuester’s high school nemesis (Neil Patrick Harris) causes trouble for the glee club. Meanwhile, Rachel struggles with a personal issue. Elsewhere Artie goes on an adventure in his dreams.

Few hours after original series aired on TV, online streaming video for Glee Season 1 Episode 19 usually available on Net and of course, if video available, as soon as possible will show you where you can watch this episode for free.