182 COLOR-BLINDNESS IN ITS RELATION TO 



These advantages are wanting in the other colored objects suggested 

 for use. Colored paper or silk may be used when light or dark, dull or 

 bright colors are wanted ; but they both have these inconveniences, they 

 must first be cut into suitable pieces, and they are troublesome to handle ; 

 moreover, they are easily concealed from view, and it is necessary to 

 stretch them carefully on a large surface to enable them to be seen 

 without trouble. They are often glaring ; they reflect, besides their 

 particular kinds of light, a quantity of white light, which is a promi- 

 nent defect, as it misleads the color-blind, who, as we know, judge of 

 colors by the intensity of light, that is, the quantity of light, and he 

 consequently estimates differently the color of a brilliant surface, ac- 

 cording to the position in which it is found with regard to the eye, etc. 

 The paper is often colored on only one side, and this gives rise to much 

 trouble, as it is necessary to turn the pieces from one side to the other 

 to see them in their true colors. Finally, from being so much handled, 

 the pieces of paper or silk soon become tumbled and faded. 



Colored glass, which must be in pieces, is not suitable either, from 

 the fact that it is difficult to procure it in sufiiciently great variety. It 

 is besides troublesome to transport, easily broken, and finally inconve- 

 nient for using, because necessary to be held against the light of day, or 

 a luminous source, in order that the color may be seen. The advan- 

 tage ot being able to use them by any kind of light does not counterbal- 

 ance their inconveniences. 



Although these are not all the objections, the preceding will suffice 

 to i)rove the advantages of Berlin wool. All this applies equally well 

 to wafers, powders, colored solutions, spools of colored thread, pieces of 

 wood, and porcelain, especially painted for the purpose, etc.; they can 

 all be and have been employed, but none of these objects are, in every 

 respect, so well suited to our purpose as Berlin wool. 



A selection of Berlin wool is then made, including red, orange, yel- 

 low, yellow-green, pure green, blue-green, blue, violet, purple, j)ink, 

 brown, gray, several shades of each color, and at least five gradations 

 of each tint, from the deepest to the lightest. Green and gray, several 

 kinds each, of pink, blue, and violet, and the pale gray shades of brown, 

 yellow, red, and pink, must especially be well represented. The choice of 

 the material does not belong specially to our method. In fact, Seebeck 

 suggested the use of Berlin wool, which was employed by his advice and 

 still is at present. To us only belongs the credit of originating the man- 

 ner in which it is employed. According to our method, the examiner 

 selects from the collection of Berlin wool in a pile on a convenient table, 

 and lays aside a skein of the especial color desired for this examination ; 

 then he requires the one examined to select the other skeins most closely 

 resembling the color of the sample, and to place them by its side. The 

 chromatic sense of the individual is decided by the manner in which he 

 performs this task. The rapidity with which this examination is made 

 does not seem to directly correspond with the nature of the chromatic 



