VISION. 135 



or diaphragm, having two vertical and parallel slits, about three millims. 

 apart, taking care that the slits are opposite the thread. When the thread is 

 contemplated through the slits, by a normal or myopic eye accommodated for 

 near vision, two white lines are seen, which converge towards the spot in the 

 thread, for the distinct vision of which the eye is accommodated, and, after 

 crossing, diverge. If the eye were hypermetropic, the lines would not con- 

 verge even if accommodated to the utmost. 



2. Sanson'S or Purkinje's images. The relative positions of the 

 observed and observing eye, and of the luminous object, which are most 

 advantageous for the observation of the image reflected by the anterior surface 

 of the lens, are stated on page 108. The experiment must be made in a dark 

 room. It is advantageous to substitute two lights, one above the other, for 

 the single luminous object referred to in the text. 



3. Chromatism. In order to see the effects described in the text 

 (p. 109) it is advantageous to place before the eye a purple glass, which, by 

 cutting off the rays of medium refrangibility, facilitates the perception of the 

 red and blue rays. 



4- Reflection of light from the Retina (see p. 109). To see 



the eye of another person luminous, the simplest way is to interpose between 

 the observed and the observing eye a reflector, consisting of several glass 

 plates applied to each other by their surfaces, in such a position that the light 

 of a lamp placed en one side of the observed eye may be seen by it. The 

 moment that this is the case, the retina is illuminated ; and if the observed 

 eye is accommodated for distinct vision of the lamp flame, and a suitable 

 concave lens placed in front of the observing eye, a distinct image of the 

 flame is seen on the observed retina, the whole interior of the globe appearing 

 at the same time luminous. 



5. The Fovea Centralis (see p. no). (a) Fix a blackboard hori- 

 zontally at a level a little below that of the eyes. Mark a point a at one edge 

 of the board, and bring the right eye up to it, closing the other, and plant a 

 pin having a white bead for its head in the board at any distance at which it 

 can be distinctly defined. Draw on the board a semicircle having the point 

 a for its centre and passing through the pin, and plant along the circle a 

 number of similar pins, at an angular distance from each other of 5 degrees. 

 If the eye is fixed on any of these pins, it will be seen that its next neighbours 

 only are seen distinctly. 



(l>) Draw two parallel lines in white, on a black ground, each millimetre 

 wide, and separated by an interval of the same width. Place the board 

 against a wall, and fix one eye on it at a distance of five feet (ij metre, and 

 consequently 100 times as far from the crossing point as the surface of the 

 retina). In a normal eye the two lines can be distinguished at that distance : 

 if not, lessen the distance until this is the case. If the eye is myopic, a cor- 

 recting lens must be used. 



In those of the following experiments which depend on the blending of 

 retinal excitations which occurs when these follow each other in rapid succes- 

 sion, a circular brass plate which revolves on a central axis is used. It is 

 furnished with an arrangement by which its rate of revolution at any desired 

 moment can be measured. 



