E. L. Nichols — Sensitiveness of the Eye to Colors. 37 



figure 4, in which the fluctuations are very marked. The cup 

 used was very small and dense. Figure 3 shows the action of 

 a cell of ordinary unglazed paper of large dimensions. The 

 current, which was as nearly as possible of the same strength as 

 in figure 4, is perfectly uniform. Figure 1 is a photograph of 

 the action of a cell made of a paper known as parchment paper. 

 We again find a regular action. Wishing to make a slightly 

 more dense cell, one was constructed of parchment of the same 

 size as those employed above. The action of the battery with 

 this cell is shown in fig. 6. It will be noticed that very slight 

 fluctuations occur at the beginning, and in parts of the line. 



Figure 7 shows the action of a cell presenting a large 

 surface, but made of very dense earthenware. In this way 

 great advantage is gained, for if the cell had been of the 

 ordinary size, its fluctuations would have resembled those in 

 figure 4. 



We can, therefore, say that there are two causes of irregu- 

 larity in the action of galvanic batteries, and that both difficul- 

 ties are overcome by making a partition of as large surface 

 dimensions as possible, and by using very porous material. 



Jefferson Physical Laboratory. 



Art. "V. — On the Sensitiveness of the Eye to Colors of a Low 

 Degree of Saturation ;* by Edward L. Nichols, Ph.D. 



[Read at the Philadelphia meeting of the American Association for the Advance- 

 ment of Science.] 



Every one who has had occasion to mix colors has noticed 

 that an exceedingly small amount of any pigment will impart 

 its hue to a very large quantity of white. One part of red 

 lead, for instance, will color a million parts of a white powder 

 like the carbonate of magnesium, and even a smaller proportion 

 than that is distinguishable by the average observer, as will 

 appear from the experiments to be described in this paper. 



This observation is strikingly at variance with the results 

 obtained by other methods of mixing colors. It has been 

 shown, for example, by Aubertf that a disk, less than -^^ of 

 which is painted (radially) with any pigment, the remainder 

 being white, cannot when in rotation be distinguished from an 

 entirely white disk. 



We have attempted to measure the sensitiveness of the eye 

 in this respect, by determining the smallest proportion of various 

 coloring matters which, when mixed with a white powder, will 



* This is one of a series of researches on the special senses by E. H. S. Bailey 

 and E. L. Nichols. 



f Rood : Modern Chromatics, p. 39. 



