One of the most important features of lead chalcogenides is the narrowing of the energy band gap as the temperature of the crystalline lattice falls, since the energy band gap widens upon cooling for most known semiconductors. For the quantum dots of lead sulfide studied in this work, the point of the first optical transition, which depends on the sample temperature, changes as the size of quantum dots is varied. Such temperature dependences are shown using samples grown in silicate glass with different average sizes of quantum dots.