1876.] . ^'-^ [Hartshorne. 



My experiments with sun-spectra, designed for the purpose of examining 

 more particularly the above facts parenthetically mentioned by Helniholtz, 

 were as follows: At about three o'clock on the afternoon of a clear day I held 

 between my eyes and the sun, a pane of colored glass; using, at intervals, 

 successively, four colors; blue, green, red and orange. After a few seconds, I 

 turned my eyes from the glass upon a white-washed wall. In each case, a 

 strong complementary (so-called negative) color spectrum was seen upon 

 the wall; Ijut, on closing my eyes, an almost equally intense positive spec- 

 trum, having, that is, the same color as the stained glass ju.st looked through, 

 appeared. Opening my eyes, the complementary spectrum returned; again 

 closing tliem, the positive one; and so on, for half a dozen or a dozen times 

 in succession. A member of my familj^ and, on another occasion, Prof 

 E. J. Houston, repeated this observation, with the same results; and I also 

 did so, with entire success, with a magnesium light, at night. These ex- 

 periments seem to me quite fatal to the supposition that retinal fatigue can 

 account for any class of spectra such as have been above considered; for, 

 if ordinary luminous impressions produce temporary fatigue and loss of 

 sensibility, stronger impressions ought to produce still greater fatigue and 

 greater loss of sensibility, or partial color-blindness, from that cause; 

 whereas the reverse is the fact. As Helmholtz states, and as Newton found 

 in his famous experiment, which proved dangerous to his sight, very bright 

 objects, such as the sun itself, give positive spectra. In Newton's case, 

 after gazing directly at the sun, its image did not pass away from his vision, 

 whichever way he looked, for two or three days. Looking through colored 

 glass, my daughter obtained a solar spectrum which continued for nearly 

 twenty-four hours. In my experiments just mentioned, if it were possible for 

 fatigue to account for the negative or complementary spectrum seen with 

 open eyes, what conceivable relation (I certainly think none) can such a 

 cause have, to the positive spectrum seen when the eyes were closed f If 

 fatigue might take aioay, so to speak, the capacity to see green light when 

 the eyes were open, it is against every "law of parsimony" in science to 

 suppose that the same cause could confer the capacity to see a green image 

 or spectrum, when the eyes were s?mt. 



Another experiment, decisive against the fatigue theory, is one by which 

 a derived or secondary spectrum may be obtained, as follows: a'small square 

 or circle of white paper or muslin is placed in the middle of a large brightly 

 colored ground, red, for instance; and is steadily looked at for from a 

 quarter to half a minute. The white central spot will acquire a tint of the 

 complementary color (green, upon red), increasing in depth with prolonged 

 attention, and especially strong at the nearest point of distinct vision. 

 Then let the eyes be turned to a white ground; there will appear upon it 

 the usual complementary (in the case supposed, green) spectrum, of the 

 ground; but, at the centre, there will be also a c\esir positive (red) spectrum, 

 corresponding in place and size, with the loMte central patch. Here, of 

 course, that part of the retina which had received only wMte lighC, could 

 not, on the fatigue hypothesis, be supposed to lose its sensibility to green, 



PROC. AMER. FHILOS. SOC. XVI. 97. 2C 



