1890.] MICROSCOPICAL JOUBKAL. 137 



with the radiating fibres of the fovea centralis^ '"^lay be rendered visi- 

 ble by the experiment in question. 



To proceed then with the details, take a microscope with the object- 

 ive and ocular in place, and the mirror or condenser so adjusted as to 

 give a moderately illuminated uniform field. Then grasp the micro- 

 scope by the standard and look steadily at the field, at the same time 

 shaking the microscope back and forth through a distance of perhaps 

 one-tenth to one-eighth of an inch. The motion may be right and left 

 or to and from the observer, or in any other convenient direction, but 

 in any case it will be seen that the portion of the network which is thus 

 made apparent consists of vessels running in a direction nearly at 

 right angles with the direction of the motion. That is, not all of the 

 network can be seen at the same instant, though a good idea may be 

 obtained of it from these successive ghmpses. 



The same general eftect may be produced by keeping the microscope 

 still and shaking or nodding the head. In fact, the conditions necessary 

 for the manifestation of this vascular net-work are, ist. A uniform field 

 from which the rays converge to a focus and then diverge to the eye so 

 that the focus really serves as a very small source of light for the eye. 

 3d. A relative motion between the eye and this focus or field. 



In appearance the net-work is dark on a light ground, no coloration 

 being apparent. The manifestation is, in fact, the shadow of the vas- 

 cular net-work thrown on the sensitive layer of the retina. Inordinary 

 vision the pupil is filled with light and measurements of the diameters 

 of the pupil and retinal vessels, and of the distance between the pupil 

 and the vessels, and between the vessels and the sensitive layer, show 

 that under such circumstances no distinct shadow is possible. The cone 

 of the umbra falls within this layer, and, therefore, it is aftected by the 

 penumbra only, and this eftect is not noticeable. With the smaller source 

 of light, however, the cone of the umbra becomes much longer and 

 reaches the sensitive layer, producing there well-defined shadows of the 

 vascular net-work. The diftbrence between the light and shade is, 

 however, not great, and due to the well-known law of the decay of 

 nervous excitement, the parts exposed to the light soon lose their greater 

 excitation and the field again appears uniformly illuminated. We do 

 not, therefore, observe this appearance when looking steadily at the field 

 of the microscope, or when there is no relative motion between the field 

 and the eye. If the head or field is moved, however, bringing about 

 such relative motion, the shadow will fall on another part of the sensi- 

 tive layer, a part w hich is relatively fatigued. As long as this condition 

 lasts, the shadow will become apparent, dark on a light field. Qiiickly, 

 however, the parts screened by the shadow^ gain in sensitiveness while 

 the others lose, and the field again appears uniform. A continual suc- 

 cession of short, quick movements will give, therefore, a continual suc- 

 cession of glimpses of this vascular shadow, giving in their entirety a 

 very gootl idea of its structure. 



It is also true that when the shadow is suddenly shifted the parts be- 

 fore screened are as suddenly exposed to the full illumination. These 

 shaded parts are relatively supersensitive, and, therefore, as the full 

 illumination falls on them they will be more eftbcted than the surround- 

 ing parts, and, therefore, the vessels will be outlined in light on a dark 

 field. Usually this eftect is not as well marked as the other, so that as 



