LXXIII.] 



PERIMETRY, IRRADIATION, ETC. 



345 



the pricks on the chart are joined by a continuous line, when we obtain an 

 oval field more extensive in the outer and lower portions. Test, if desired, the 

 left eye, substituting a blank chart for that eye. 



(,/. ) Test the field of vision for colours, substituting for the white travelling 

 disc blue, red, and green. Mark each colour-field on the chart with a pencil 

 of similar colour. Notice that the field for blue is nearly as large as the 

 normal visual field. It is smallest for green, red being intermediate between 

 green and blue. 



(</.) With Ludwig's apparatus test when red, yellow, blue, and other 

 coloured glasses cease to be distinguished as such in the field of vision. 



2. Binocular Vision. 



(a.) Hold in front of each eye a blackened tube. On looking 

 through both tubes two fields will be seen. Gradually cause the 

 tubes to converge at their free ends, and the two fields of vision will 

 be seen to meet and form a single field. 



(/;.) Continue the convergence, and note that two fields reappear, 

 but they are crossed. In these " secondary positions " there is no 

 rotation of the eyeball on its antero-posterior axis. 



('.) If the eyeball be turned in any other direction (tertiary 

 positions) the after-image appears inclined, or at an angle with the 

 vertical or horizontal stripes, according to the original position of 

 the red fixation-object. 



3. Wheel Movements (False) of the Eyeballs (Secondary and Tertiary 

 Positions). 



(.) On a grey sheet of stout paper, at least I metre square, rule a number 

 of vertical and horizontal faint 

 black lines. Fix on the centre of 

 the paper a strip of red paper on a 

 level with the eyes, the eyes being 

 in the primary position, i.e., look- 

 ing straight ahead. Gaze steadily 

 at the latter, keeping the head 

 fixed. After a time suddenly direct 

 the eyeballs to another part of the 

 grey surface ; a green-blue after- 

 image is seen which retains its 

 same relative position with regard 

 to the vertical and horizontal lines, 

 provided the eyeballs be moved 

 directly upwards, downwards, in- 

 wards, or outwards, i.e., if the eye- 

 ball is moved up, along vertical or 

 horizontal meridians, the after- 

 image is still vertical. Turn the 

 eyeball upwards and to the right, 

 or downwards and to the left, the 

 head being kept in the same posi- 

 tion, the after-image appears tilted 

 to the right ; if the eyes are directed 

 upwards and to the left or downwards and to the right, the after-imgae appears 

 tilted to the left. A similar result occurs with a horizontal strip of paper, 



FIG. 263. Appearance of a Cross in False 

 Wheel Movements of Eyeballs. 



