■June 2, 1899.] 



SCIENCE. 



773 



at the windows. The windows, 1, 2 and 3, 

 are, howevei', fitted witli gray glasses. ISTo. 

 1 carries a very darlj smoked glass ; all 

 colors seen throngh it will be dark. No. 2 

 carries a piece of ground glass, showing all 

 colors in full brightness. No. 3 carries a 

 light smoked glass. There are thus thirty- 

 six possible combinations of the colors. The 

 twelve glasses are, however, mainly reds, 

 greens and grays. 



A suitable arrangement of the colors 



reds, greens and grays simultaneously in a 

 large number of different shades of inten- 

 sity. The light of a green lantern, at dif- 

 ferent distances or in a fog, is simulated by 

 the green behind the different grays ; at the 

 same time a white light is also changed. 

 The color-weak person to whom weak green 

 is the same as gray (white at a distance) is 

 utterly confused and thinks that the weak- 

 ened green is gray (white) and the dark 

 gray is green. 



gives direct simultaneous comparisons of 

 reds, greens and grays, of different shades. 

 The well-known confusion by color-blind 

 persons of dark greens with reds, greens 

 with gray, etc., are exactly imitated, and 

 the instrument gives a decisive test for 

 color-blindness. Its peculiar advantage, 

 however, lies in the fact that it presents 



The actual test is performed in the fol- 

 lowing manner. The tester is held toward 

 a window, at about 2\ feet from the person 

 tested. The operator begins with any 

 chance position of the glasses, and asks the 

 person tested to tell the colors seen through 

 the three glasses, Nos. 1, 2 and 3. He an- 

 swers, for example : " No. 1 is dark red ; No. 



