Определить время) 1) the problem was first recognized in the 19th century. 2) later an idea was suggested to apply it to practical things. 3) many possibilities for practical applications were analyzed. 4) some of these were tested by experiment. 5) the experimental results were not generally accepted, and the idea was discarded. 6) then other consequences were deduced and a new model proposed. 7) recently the model has been modified and is now being used in many practical situations.
7) Present Perfect Passive