log  X = log  X / log  b1
   b1       b2       b2

Or,

log  X = C * log  X
   b1           b2

So,

log  Y = C * log  Y
   b1           b2

And,

log  X / log  Y = C * log  X / C * log  Y  
   b1       b1           b2           b2 

                = log  X / log  Y
                     b2       b2

Therefore, the ratio of the logs of two numbers is the same,
regardless of the base used.