STIMULATION OF THE RETINA 123 



modafcing for distance. In a minute or two a number of branching 

 figures will be seen like the roots of trees. These are the shadows 

 of the retinal blood-vessels. When the light is moved the shadows 

 are also seen to move. This experiment shows that the receptive 

 visual cells lie behind the blood-vessels of the retina. 



Perimetry. The perimeter is an instrument for testing and re- 

 cording light perceptions in different parts of the retina. The eye is 

 fixed upon a point in the centre of the concave hemisphere forming the 

 perimeter, and a white disc is brought gradually from the edge of the 

 hemisphere nearer and nearer to the centre until it is perceived. This 

 is repeated along different meridians, and the results are marked on a 

 chart at the back of the instrument. By using coloured discs instead 

 of white the area of the retina which is sensitive to each colour can 

 also be ascertained. 



Testing for colour vision. Edridge-Green's lamp and spectroscope. 

 Holmgren's wools. The best practical method for testing colour 

 vision is by the use of a lamp provided with glasses of different colour, 

 the subject being expected to name the colour which is exhibited. A 

 more accurate method of obtaining spectral colours for testing purposes 

 is the employment of a spectroscope so arranged that only a definite 

 part of the spectrum with a pure spectral colour is visible at one time. 



A method which has been considerably used for testing colour 

 vision is to take a box full of skeins of wool dyed with different colours, 

 and, selecting one skein, to ask the person who is being tested to pick 

 out any that match it. If he is colour-blind he is liable to make 

 serious mistakes, matching grey with red, green with grey or red, and so 

 on ; but it occasionally happens that persons who fail completely with 

 the lamp test are able, probably by judging from the intensity of the 

 reflected light, to match the Holmgren wools fairly well. 



Successive contrast. Fix the vision upon a white spot on a dark 

 ground. After one minute look at a uniform white surface such as a 

 white ceiling. A dark spot now occupies the centre of the field of 

 vision. 



This experiment is varied by employing colours e.g., a yellow 

 spot on a blue ground or vice versa ; and a red spot on a green ground 

 or vice versa. In each case the vision is transferred to a uniform white 

 surface and the contrast colours are observed. 



For the grounds, coloured paper is used ; for the spots, either discs 

 cut out of the paper, or wafers laid upon it. 



Meyer's experiment. Place a grey disc upon a yellow ground, and 

 cover the whole with thin tissue paper : the grey disc at once appears 

 blue ; the contrast colour of the yellow. If a blue ground be used the 

 disc will appear yellow. The same experiment may be repeated with 



