Fitting disc brakes

pk64 Posts: 53
edited April 2011 in MTB beginners
I am trying to work out what adapters etc I need to fit disc brakes to my bike that currently only has V's but I am now really confused because when I tried to fit an adapter from my dad's bike it wouldn't fit.

This is the rear drop out on my bike that will not accept the adapter...

... and this is the rear drop out on my dad's bike that the adapter came off.

So, does this mean that I cannot fit disc to my atall or does it just mean that a different type of adapter is needed?

Thanks in advance for any help, Peter.
2009 Trek 3900 - MTB (Upgraded)
1976 Eddy Merckx - Road (Restored)