In An Essay Towards a New Theory of Vision, published in Dublin in 1709, there was a passage like this:
Although mathematicians sought to explain the perception of distance through lines and angles, those lines and angles themselves could not be seen. Nor did those unskilled in optics ever think of them. Most people did not carry rangefinders around with them, nor, after seeing an object, did they calculate its distance by the angle formed between two optical axes.
Everyone could judge what they themselves perceived and what they did not. If we were not conscious of certain lines and angles producing various notions of distance in our minds, then even if every mathematician in the world told me I perceived them, would that not be pointless?
If an idea itself could not be perceived, then we could not use it to perceive other ideas.
The author of that book was George Berkeley, an opponent of Isaac Newton. When Newton and Leibniz founded calculus, turning it into an important tool for solving countless problems, Berkeley attacked Newton with the logically rigorous question of whether an infinitesimal was ultimately zero.
For example, if two mirrors were placed facing each other with a doll between them, countless dolls would appear. This experiment could easily be done at home. So were there a finite or infinite number of images in the mirrors?
From the perspective that light was energy, after being reflected too many times, its intensity would fall until it could no longer form an image.
Suppose light suffered no loss through repeated reflections between the two mirrors. Then as the number of reflections increased, the doll would grow smaller and smaller, until it became too small for the human eye to observe. Did an image that the human eye could not