"Between the electric light and the photo-electric cell is placed a glass vessel holding an alkali that is to be neutralized. Above is a tube from which an acid passes, drop by drop, through an automatic valve, into the alkali. A small amount of chemical indicator added to the alkali maintains a red color in it until it is neutralized. When a sufficient amount of the acid has dropped into the alkali, the red color disappears, indicating complete neutralization.
"When the solution is colored red, an insufficient amount of lights gets through to the photo-electric cell. As the red color gradually diminishes, the amount of light passing through increases, and when the solution is entirely clear the light reaches a critical value which causes the photo-electric cell to pass a signal to the radio tube. This tube operates the relay which closes a valve and shuts off the supply of acid.
"Using a device of this sort to perform such operations around a laboratory will save a great deal of a chemist's time. Its electric eye is about 165 times as sensitive to differences in color as any human eye."
The fly landed with a thud on the center table.