How do you balance a portfolio based on percentages?

Are you supposed to balance it based on the money you put into a specific asset or the amount that the asset is currently worth?

Say you invested $100 as follows:

50% stocks 50% bonds

$50 stocks and $50 bonds

If the stocks fall and are now worth $5 less you have:

$45 stocks and $50 bonds

If you add $10 to your portfolio which are you supposed to do:

+$5 stocks and +$5 in bonds

So that the amount you invest in each is 50%.

OR

+$7.50 in stocks and +$2.50 in bonds

So that the value of your stocks and bonds are each worth 50% of the total value.