When this condition is reached, the angle of incidence is called the critical angle.
For diamond this critical angle, as it is called, is very nearly 24° from a perpendicular to the surface.
The question is, is the critical angle really as high as Mr. Seller's experiments would show.
There are certain niceties which might be introduced, such as the "critical angle of the glass."
The heavy line indicates what is commonly known as the critical angle.