MONOPOLY 
103 
been admitted with advantage into the 
principles of payment for certain public and 
semi-public services. Doctors, for instance, 
frequently charge patients of different classes 
on different scales. 
Jt now remains to discuss the principle 
applied in the settlement of discriminative 
charges, when they are possible. It is 
exactly the same as the principle of the 
single monopoly charge, but its application 
is more complicated. The monopolist tries 
to fix the two or more prices which he finds 
practicable at such levels that his net returns 
are maximised. We may think of the 
monopolist as dealing in two markets. In 
each market he fixes the price which will 
be the most profitable, regardless of the 
price obtained in other markets, except in 
so far as the quantity of sales in both 
markets taken together must be considered 
because of its bearing on cost of pro 
duction. It will be instantly realised how 
exceedingly improbable it is that the 
most profitable price in the one market 
should be identical with the most profitable 
price in the other. In the one it might 
pay to sell a large quantity at a low price, 
whereas in the other, in view of the different 
circumstances of the community served, it