Invented? Hardly, variable focus spectacles have been done many times before.
<a href="https://www.google.ca/search?q=variable+focus+spectacles&oq=variable+focus+spectacles&aqs=chrome..69i57&sourceid=chrome&ie=UTF-8" rel="nofollow">https://www.google.ca/search?q=variable+focus+spectacles&oq=...</a><p>This can easily be done with an autofocus mechanism, as used in cell phones to adjust a variable fluid filled gap (which has been done many times before)
The biggest problem in the past was the weight, lens thickness and leakage of the liquid. They could also use an air gap, but that is even thicker.