[turnbull] a subjective PHENOMENON OF VISION 29 



Supplement to paper on A Subjective Phenomenon of Vision. 

 By W. EuPERT Turnbull, M.E. 



June 22nd, 1904. 



The more thorough investigation alluded to in the first part of this 

 paper was carried out at Cornell University, New York State,; from 

 October, 1S96, to December, 1896. With a very accurate chronograph 

 and methods of obtaining pure spectral colours the experiments described 

 in the foregoing paper were very much extended; but the same general 

 results were obtained and the work as a whole may be summarized so 

 is to include both series of experiments. 



A further series of experiments carried out in May and June, 1904, 

 shows that the oscillations arc quicker when a near object is " fixed " 

 than when a distant object is " fixed," the periodicities in the two cases 

 being -544 seconds and -595 seconds, respectively. 



A summary of the chief phases of the phenomenon in all experi- 

 ments, from January, 1895, to June, 1904, is as follows : 



1. The writer, as a boy, perhaps fifteen years before these experi- 

 ments were begun, saw in the dark a subjective phenomenon of vision, 

 consisting of hexagonal figures (see Fig. 1), composed of more or less 

 distinct " light dust." These figures oscillated as a whole and then 

 streamed from the field of vision. 



2. From time to time in the following years these figures were 

 occasionally seen, and in 1895 it occurred to the writer that it would 

 be interesting to study these figures and their oscillations. 



3. The more they were studied the more easily they could be 

 recalled, until now, with certain conditions of background, they may be 

 seen at any time, with eyes either open or shut, and the vibrations 

 may be counted — with open eyes the image appears projected on the 

 focussed object. 



4. The form of the hexagonal figures as first seen and as often 

 seen in the dark at present remind one very strongly of the illustrations 

 of the magnified crystalline lens cells. 



5. The retinal size of the image when light enters the eyes is 

 about -183 mm, whereas the size of the Fovea centralis (the point of 

 accurate vision) is from -IS mm. to -225 mm., therefore the image is 

 seen only at the point of accurate vision. 



6. "When light does not enter the eyes the retinal size of the 

 image is about -47 mm. 



7. The amplitude of the vibrations is about one-tenth the size of 

 the image. 



