ELEMENTAEY EXPERIMENTAL PHYSIOLOGY 243 



The visual acuity diminishes rapidly on the retina as we recede from 

 the fovea. The diminution is more marked in the vertical than in the 

 horizontal meridian.] 



[ADVANCED EXPERIMENT. Place on a card two dots, each 2 mm. in 

 diameter and separated by a distance of 2 mm. Let the gaze be fixed 

 on a mark on a vertical white sheet of paper, and let the card be moved 

 in a horizontal meridian gradually nearer the mark till the two dots can 

 be discriminated. Compare the vertical and horizontal meridia in this 

 respect.] 



The acuteness of vision at the fovea is ordinarily tested by noting the 

 distance at which letters, which at a given distance subtend an angle of 

 5' at the eye, can be read. This method may be applied either to 

 ascertain what error of refraction may exist in the eye, or if this be 

 absent or corrected, what the acuteness of vision in the particular eye is. 



EXPERIMENT III. Using Snellen's or Jaeger's test types, ascertain 

 whether the letters can be correctly named at the normal distance in a 

 good light. If this distance can be exceeded or if it cannot be reached 

 an expression for the condition of the acuteness of vision may be 

 written as follows : 



where d = distance of person from the types and D = number of smallest 

 type which a person can read at that distance. 



6. Mechanical Stimulation of the Retina. Phosphenes. The 

 retina can be stimulated by pressure on the sclerotic. An image will 

 be produced which is referred to the opposite portion of the field 

 of vision. 



EXPERIMENT I. Close one eye and turn it as far as possible towards 

 the nose. Press with a pencil point on the sclerotic, through the eye- 

 lid, at the edge of the orbit on the outer side. Note the circle of light 

 which appears on the nasal side. The retina is stimulated just beneath 

 the pressure and the image is referred to the nasal side of the field of 

 vision. 



7. The apparent Inversion of Shadows thrown upon the Retina. If 

 a beam of light falling upon the retina be intercepted by some object 

 close to the cornea, an erect shadow of the said object will be thrown 

 upon the retina. This, however, will be projected into the field of 

 vision as an inverted image. 



EXPERIMENT. The Experiment No. 6, Section I. in Milton Bradley 

 Pseudoptics, illustrates the nature of retinal shadows well. 



8. The Perception of Colour in the Peripheral Portion of the Retina. 

 The sensibility of the retina for colour varies in different zones of the 



