How does the author use imagery in the story, There Is a Wolf in My Time Machine?

One of the notable traits of Niven's fiction is his sharp imagery of amazing people, places, and machines. For instance, in There Is a Wolf in My Time Machine, he describes what the extension cage looks like from the outside when it is in operation: The extension cage rested a few yards away, the door a black circle on one curved side. The other side seemed to fade away into nothing. It was still attached to the rest of the time machine, in 1103 PA, along a direction eyes could not follow. This is stuff to excite the imagination.