Experimental physiology / by Sir Edward Sharpey-Schafer.

  • Edward Albert Sharpey-Schäfer
Date:
1927
Catalogue details

Licence: In copyright

Credit: Experimental physiology / by Sir Edward Sharpey-Schafer. Source: Wellcome Collection.

    146/156 page 134
    one side of the eye. Look steadily at the paper with one eye, accom- modating 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. The presence of the vessels may also be appreciated by moving the field in relation to the retina. Look at the sky through a pinhole in a sheet of card- board, give the card an up and down, side to side, and lastly a rotatory motion, and the branching shadows will be perceived with a central free space corresponding to the macula. The retinal vessels can still better be seen by using a very small electric bulb mounted on a stem. The bulb is pressed directly backwards on the lower eyelid of the closed eye, and moved about slightly; in a certain position, which has to be found by trial, the observer will see his own retinal vessels standing out as red streaks on a darker red ground. The vessels disappear into a dark area, the blind spot. In carrying out this experiment it is best to close both eyes. The red corpuscles moving in the retinal vessels can be seen by looking at a bright sky through a piece of dark-blue glass. After a few seconds the field of vision exhibits minute silvery spots in rapid movement; these are caused by the shadows of the red corpuscles within the retinal vessels falling on the light-sensitive rods and cones. 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
    No text description is available for this image
    No text description is available for this image
    No text description is available for this image