COLOR-VISION AND COLOR-BLINDNESS. 703 



averages about a minute a person. I have heard and read of instances 

 of color-blind people who had i)assed the wool test satisfactorily, and 

 had afterwards been detected by other methods, but I confess that I do 

 not believe in them. I do not believe that in such cases the wool test 

 was applied properly, or in accordance with Holmgren's very precise in- 

 structions; and I know that it is often applied in a way which can lead 

 to nothing but erroneous results. Railway foremen, for example, re- 

 ceive out of a store a small collectiou of colored wools selected on no 

 principle, and they use it by pulling out a single thread, and by asking 

 the examinee, " What color do you call that ?" Men of greater scien- 

 tific pretensions than railway foremen have not always selected their 

 pattern colors accurately, and have allowed those whom they examined 

 and passed to make narrow comparisons between the skeins in all sorts 

 of lights in a way which should of itself have afforded sufficient evi- 

 dence of defect. 



Although however the expert may be fully satisfied by the wool test 

 that the examinee is not capable of distinguishing with certainty 

 between red and green flags or lights in all the circumstances in which 

 they can be displayed, it may still remain for him to satisfy the employer 

 who is not an expert, the railway manager, or the shipowner, and to 

 convince him that the color-blind person is unfit for certain kinds of 

 employment. It may be equally necessary to convince other workmen 

 that the examinee has been fairly and rightly dealt with. Both these 

 objects may be easily attained by the use of slight modifications of the 

 lights which are employed. Lanterns for this special purpose were 

 contrived some years ago by Holmgren himself and by the late Pro- 

 fessor Bonders, of Utrecht, and what are substantially their contriv- 

 ances have been brought forward within the last few months as novel- 

 ties by gentlemen in this country who have re-invented them. The prin- 

 ciple of all is the same, namely, that light of varying intensity may be 

 displayed through apertures of varying magnitude and through colored 

 glass of varying tints, so as to imitate the appearances of signal lamps 

 atdiflPerent distances and under difterent conditions of illumination, of 

 weather, and of atmosphere. To the color-blind the difference between 

 a red light and a green one is not a difference of color, but of luminos- 

 ity, the color to which he is blind appearing the less luminous of the 

 two. He may therefore be correct in his guess as to which of the two 

 is exhibited on any given occasion, and he is by no means certain to 

 mistake one for the other when they are exhibited in immediate suc- 

 cession. His liability to error is chiefly conspicuous when he sees one 

 light only and when the conditions which govern its luminosity depart 

 in any degree from those to which ho is most accustomed. With the 

 lanterns of which I have spoken it is always possible to deceive a color- 

 blind person by altering the luminosity of a light without altering its 

 color. This may be done by diminishing the light behind the glass, by 

 ncreasiug the thickness of the red or green glass, or by placing a piece 



