Transparency Effect

I have a question regarding this >  icon…how do I get that transparency effect in the middle between those two pictures of Brass and Cathrine? I use photoshop v.7, and what I thought I had to do was make the locked background layer transpaprent….but when I did that my icon turned out like this  >  (if your journal has a white background, you won´t see the difference, but my computer substituted the transparent part with white) Anyone got an idea?