In the early years of rail transportation, thousands of people injured or killed in railway accidents were thought to be victims of carelessness. But in reality, the train conductors may not have been able to distinguish the safety signals. In 1876, a Swedish train accident prompted Dr. Fithiof Holmgren to test all 266 railway employees for color blindness. He found that 13 of the employees were colorblind — including the engine driver.
This discovery spurred color blindness screenings for all railway employees around the world using a test originally devised by Dr. Holmgren and later simplified by Dr. William Thomson. The Mütter Museum of The College of Physicians of Philadelphia shared an image on Figure 1 of Dr. Thomson’s test, which comprised numbered and colored yarn.
The box included directions for the use of Dr. Thomson’s test.
Directions according to Dr. Thomson’s original test:
- “Spread a white cloth, like a towel, on a table in good light.”
2. “Take out all the [pieces of yarn] from the green part of the box and put them on the cloth at random.”
3. “Take from this heap the large light green [piece of yarn]…and laying it to the side, direct your man to select 10 [pieces of wool] from the heap. Tell him that these are not to be exactly like [the light green piece] in every respect, but that they are to be of the same color, only a little lighter or darker in shade.”
4. “Write down on your [paper] the numbers of the 10 [pieces of yarn] selected by your man to match the [light green piece]. If only odd numbers appear which he selected promptly, then he is not color blind; but if even numbers have been chosen, he must be more or less color blind.”
5. “Ask him the name of the color he has been matching…and register his answer on the blank [paper].”
6. “Remove now all the [pieces of yarn] and put them back.”
7. “Now take out [the next set of yarn pieces] and expose it on the table in the same way as before.”
8. “Show him the large rose-colored [piece of yarn], and ask him to match this with 10 [pieces of yarn] in the same sense as before with the first test.”
9. “Register his 10 selections on the blank. Now any even numbers…determine definitely his color blindness. If your man selects blues, he is red-blind; if he selects grays or greens, he is green-blind.”