[turnbull] a subjective PHENOMENON OF VISION 17 



and at the ends of it reflected the light from the burner through the 

 holes. A piece of plate glass was mounted vertically a little above the 

 middle of the box outside. A certain amount of light, from each hole 

 was tlius permitted to enter the eye in the same direction, a part of the 

 light from one hole being transmitted through the glass plate and a part 

 of the light from the other hole being reflected by it. The intensity of 

 the light passing through the respective holes was varied by turning the 

 reflectors about a horizontal axis, the angles through which they were 

 turned being read on graduated arcs placed at each end of the box outside. 



RESULTS OF OBSERVATIONS. 



The retinal size of image and the amplitude of oscillations. 



23. Although the border of the image is not sharply defined, still, 

 fairly accurate measurements of the retinal size of the image and rather 

 unsatisfactory estimations of the amplitude of vibration, have been ob- 

 tained. 



34. Pieces of white paper were used, upon which circles of 5, 10, 

 15, 20 and 25 mm., respectively, were drawn with black ink. Placing 

 these pieces of paper in succession before my eyes I retreated or ad- 

 vanced, with eyes fixed upon the centre of the circle, until the image 

 appeared to completely fill the circle; then the distance from the eyes 

 to the paper was measured — these experiments were carried on in a 

 brightly lighted room with the observer's back turned towards the light. 



25. A calculation of the retinal size of the image (from these data) 



f d . . 



is simple, from the proportion : — — ^ = — ; when / is the distance of 



the eyes from the baclvground ; F the distance of the second nodal point 

 of the cr}'stalline lens from the retina (this is, as lan average, 15 mm.), 

 u the diameter of the image upon the background, and D the corres- 

 ponding diameter upon the retina. An average of the observations 

 taken (5 sets of 5 readings each) gave as the value for D, the retinal 

 diameter of the image .0-183 mm., when light is entering the eyes. 



26. The diameter, as gdven by Kolliker, of the Fovea centralis of 

 the retina is from 18 mm. to -225 mm. As the image is invariably 

 seen at the point of direct vision, and as the above value (-183 mm.) 

 falls within the measurements of Kolliker, the image nndoubtedlv is 

 seen only by the rods and cones of the Fovea centralis (this when light 

 enters the eyes). 



27. That more of the image is seen when no light enters the eyes 

 is shown by the following experiment: 



Sec. III., 1906. 2 



