Casimir derived his equation to a fourth order perturbation. This amounted to a reasonable approximation for the force, but it was not precise.
In the Aether Physics Model I developed the magnetic force law for the electron. Later, I discovered the magnetic force law for the electron was nearly identical to the Casimir force equation, although transposed into different units and constants.
Just today, it was brought to my attention that Hendrick Casimir's paper published in Phys Rev 73, 360 is now available online. Here is the last paragraph of that paper:
"The very simple form of eq (56) and the analogous formula (25) suggest that it might be possible to derive these expressions, perhaps apart from the numerical factors, by more elementary considerations. This would be desirable since it would also give a more physical background to our result, a result which in our opinion is rather remarkable. So far we have not been able to find such a simple argument."Casimir had been looking for the simple expression for his equation, but was unable to find it. The reason was because modern physics incorrectly notates the dimension of charge (it should always be distributed) and also modern physics did not discover the proper quantification for magnetic charge (strong charge, electromagnetic charge). The APM fixed these errors of modern physics and solved the Casimir Problem.