Towns may grant pensions to any fireman, police officer, or constable, who, by reason of permanent disability directly incurred in the performance of his official duty, is no longer able to perform services in such capacity, or who has served faithfully for not less than 25 years; provided that no pension shall be granted for more than one year at a time. The maximum amount of such pension shall be in the case of a permanent man
1/2 of the pay received by him at the time of his retirement or disability, and in case of a part-time man, call man or special man, $2,500.