When holding glass in first person the block is transparent to all things in the world. The same is true in third person except for some edge cases listed below:

• water (source and flowing)
• clouds.
• slime blocks
• glass panes
• glass blocks
• honey blocks

When viewing the object through the glass block you're holding in third person these blocks/things are invisible. 

It should be noted this only affects third person view, first person appears to work for all listed cases.