What technological advancement was necessary before scientist could begin to observe cells?

The technological advancement that was needed to be made before scientist began to observe cells was the invention of the microscope. I have answered this question before on here

what have scientists found to be the most efficient and effective way to insert new sequences into an individual? a-scientists have created genetically altered foods that release dna during digestion. b-scientists use viruses to insert specific gene sequences. c-scientists irradiate genes so that they can be absorbed through the skin.