Hartshorne.J -^-t) [April 21, 



On turning tlion to a irhife ground, most of the retina beins; charf/ed iritJi 

 red by the previously contemplated red ground, the usual complementary, 

 green, spectrum of that ground is seen; but at the central part of the retina, 

 whose green-re»onating elements have been set in vibration (as above said), 

 their vibrations are annulled by the green /'rtj/sof the white ground; leaving 

 only the red rays of that ground to take effect, and producing a (secondarj') 

 red spectral spot upon the green. 



These facts are, it appears to me, all closely similar in nature to the 

 commonly obsersx'd interference occurring between diffused sunlight 

 and lamp or candle flame; and, also, they have a definite relation to the mode 

 of production of the Fraunhofer dark lines or "absorption bands" of the 

 spectroscope; while they suggest some further speculations in regard to 

 what may be the molecular conditions of reflecting surfaces which give to 

 our vision impressions of different colors, to enter upon which, at present, 

 would extend this communication to too great a length. I may simply add 

 that, on careful examination of ocular spectra obtained by looking at 

 colored objects by lamplight, as well as by sunlight, I have become con- 

 vinced that the influence of light entering through the closed lids, if they 

 are uncovered and within range of a luminous source, such, even, as that 

 of a student's lamp, is not at all indifferent. A spectrum, obtained as usual, 

 seen with closed eyes, and fading away while thcj' are covered, ma\- be 

 renewed by approximating the eyes, still closed, to the light; and this again 

 and again, several times. In order to test yet farther the sensitiveness of 

 the eye, when closed, to light, I placed several pairs of glass slips, of differ- 

 ent colors (blue, red, green and orange), in the hands of an assistant; lay- 

 ing them, then, as given to me, in turn, upon my closed lids, while my 

 eyes were turned towards tlie afternoon sun. In every case, I could dis- 

 cern clearly the color of the glass; but only in the direct rays of the sun; 

 diffused daylight, and lamplight f\iiled, with me, to give more than a doubt- 

 ful discrimination of color, even with near contact of bright translucent 

 objects, while the eyes were closed. 



Incidentally, I may mention also, my careful repetition of a very curious 

 experiment mentioned bj' Sir. David Brewster in his Optics, and brought 

 to mj' notice ])y Prof. E. J. Houston. A strip of white paper is held verti- 

 cally, about a foot from the eyes; the attention is then fixed upon an object 

 at a greater distance, so that the slip of paper is seen double; and at the 

 same time a lighted candle is brought near to the side of one eye, so that 

 its light will affect that eye strongly-. The image of the strip seen by tliat 

 eye (if the right eye be illuminated, the left-hand image, and vice versa) will 

 be green, and the other pale red, I find that, when these two images have 

 been obtained, one eye can be closed, but the same color still remains. Brews- 

 ter asserts that when two equal candle-lights arc used, one being held near 

 each eye, two wJiite images will be obtained. With mc, this does not prove 

 to be the case. I see, with two candles, two green images instead; and, fix- 

 ing the eyes on the strip for single vision, with two candles placed as before, 

 it is seen of a distinct green color. Without insisting upon it, I would pro- 



