Usually, a long pause is used to give the effect of suspense or dramatic effects. Another word for this is Aposiopesis.
Aposiopesis can be used to show that a character is overwhelmed with the emotion of fear and or horror arising from their imagination.
In the section mentioned above, the speaker includes a long pause in order to underscore that Mrs. Hall's horror was due to an overactive imagination.
As we learn, Mrs. Hall thinks that her visitor is a monster with a large mouth that engulfed the scientist's face. But when the scientist stirs, Mrs. Hall opens the door wider and as more light falls on the scientist's face, the muffler that held up his face becomes visible. The shadows had twisted her imagination.
Learn more about Literary devices in the link below:
https://brainly.com/question/2183813
