How is the middle of the $19^{th}$ century an important reference to photosynthesis?

(N/A) $\Rightarrow$ By the middle of the $19^{th}$ century,the key features of plant photosynthesis were known,specifically that plants could use light energy to synthesize carbohydrates from $CO_{2}$ and water.
$\Rightarrow$ The empirical equation representing the overall process of photosynthesis for oxygen-evolving organisms was understood as:
$\Rightarrow$ $CO_{2} + H_{2}O \xrightarrow{\text{light}} [CH_{2}O] + O_{2}$
$\Rightarrow$ Here,$[CH_{2}O]$ represents a carbohydrate (e.g.,glucose,a six-carbon sugar).