Chairs came into existence in various forms with addition to back support, sacral support and footstool to a normal stool.
<u>Explanation:</u>
The chairs which we use today to sit on and be comfortable were not the types how they emerged. The emergence of the chairs were in the form of stools in the first form. But then chair of different forms and sorts came into existence to increase the comfort of the human beings.
To the stools, the addition of back was done, then the sacral support was done to the chair. After some time the foot stool was also added to the chair to increase the support.
They used to use roller skates and people got more in jured
The path of light, the lens, mirror in up position, pentaprism, veiwfinder, film, shutter, mirror in down position, and diaphram
Moral is the answer. Hope that helps
