Michell has become known more recently for his letter to Henry Cavendish, published in 1784, on the effect of gravity on light.

This paper will be rediscovered in the 1970s and is now recognized as anticipating several astronomical ideas that had been considered to be twentieth century innovations.

Michell is now credited with being the first to study the case of a heavenly object massive enough to prevent light from escaping (the concept of escape velocity is well known at this time).

Such an object, which he calls a dark star, will not be directly visible, but can be identified by the motions of a companion star if it is part of a binary system.

The classical minimum radius for escape assuming light behaves like particles of matter is numerically equal to the Schwarzschild Radius in general relativity.

Michell also suggests using a prism to measure what is now known as gravitational redshift, the gravitational weakening of starlight due to the surface gravity of the source.

Michell acknowledges that some of these ideas are not technically practical at this time, but writes that he hopes they will be useful to future generations.

By the time that Michell's paper is rediscovered nearly two centuries later, these ideas will have been reinvented by others.