ELEMENTARY EXPERIMENTAL PHYSIOLOGY 107 



again are followed by slight shadows. Fix the gaze and note that 

 these points move in constant directions. They probably represent 

 small local collapsings of fine capillary blood-vessels, caused by tem- 

 porary clogging of the red corpuscles. The re-filling of the vessel 

 brings about the shadow following the bright point. 



3. The Blind Spot. A certain region of the retina, to the inner side 

 and somewhat below the macula lutea, is insensitive to light, inasmuch 

 as the optic nerve here enters the eyeball, and the layer of the retina 

 which reacts to the stimulus of light is here absent. This insensitive 

 region is spoken of as the optic disc or blind spot. 



Experiments showing the nature of the blind spot may be con- 

 veniently carried out with the material in Section H. of the Milton 

 Bradley Pseudoptics series. 



EXPERIMENT I. Using cards H.2, or H.3, close the left eye and fix 

 the gaze of the right eye on the cross. At a distance of about eighteen 

 inches the tree in H.2 or the red disc in,H.3 will disappear. 



EXPERIMENT II. Arrange the cards H.4 and H.5 at such a distance 

 that when the left eye is closed and the right eye gazes at the cross, 

 the house in H.4 or the red spot in H.5 falls on the blind spot. It will 

 be found that similarly, with the right eye closed and the left eye 

 fixating, the cross, the church, and the yellow disc will be invisible. 

 Having found the proper distance, open both eyes and place the card 

 H.4x close to the nose and in the plane of the septum of the nose. It 

 will be found that when the gaze is directed to the cross the surface of 

 the cards appears uniformly white. 



EXPERIMENT III. If a dot and a cross be drawn about four inches 

 apart, the dot being about half-an-inch above the horizontal level of 

 cross, and if then the left eye be closed and right eye gaze at the dot, 

 at the distance of about a foot, the cross will be invisible, as its image 

 falls on the blind spot. 



When any image falls upon the blind spot it is invisible. By imagina- 

 tion we Jill in this region of any image falling upon the retina by 

 sensations similar to those in the neighbouring regions. This is well 

 illustrated in the following experiments. 



EXPERIMENT IV. Using the cards H.6, H.7, H.8, and H.9, and 

 ascertaining the distance at which they should be placed, as in 

 Experiment I., notice that when the coloured discs fall upon the blind 

 spot, the place of the discs is taken by a combination of the background 

 on which the discs lie. In H.9 in particular there seems no break in 

 the chequered pattern forming the background to the red disc. 



The blind spot may be mapped out with ease in the following 

 manner. 



