Fifteen o'clock
In a space at the center , not occupied by any of these glass squares , was a dark oblong area and a ledge holding a piece of chalk . And above the area was a huge brass cylinder ; toward this brass cylinder the professor would soon direct his subsequent remarks . In order to assure himself that it was time to press the button which would notify the members of the class in history to approach their local Visaphones , the professor withdrew from his vest pocket a small contrivance which he held to his ear . Upon moving a tiny switch attached to the instrument , a metallic voice , seeming to come from somewhere in space , repeated mechanically : `` Fifteen o'clock and one minute -- fifteen o'clock and one minute -- fifteen o'clock and one min -- '' Quickly , the professor replaced the instrument in his vest pocket and pressed a button at the side of the Visaphone . As though in answer to the summons , the frosted squares began , one by one , to show the faces and shoulders of a peculiar type of young men ; young men with great bulging foreheads , bald , toothless , and wearing immense horn spectacles . One square , however , still remained empty . On noticing this , a look of irritation passed over the professor 's countenance .