ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



265 



are made in other meridia. In this manner the limits of vision in the 

 different meridia of the field of vision can be recorded. 



It is of course essential that the subject keep his eye fixed on / the 

 whole time the spot is being moved. 



The area bounded by a line drawn through the limiting points in the 

 different meridia is properly the area of the field of vision. It is, how- 

 ever, often desirable to refer this area to the retina. If the meridia be 

 inverted, the figure traced would then correspond to the sensitive 

 portion of the retina. It will be found that perimeters are generally so 

 constructed that the limiting marks in the different meridia are inverted 

 on the chart, so that the latter becomes a chart of the extent of the 

 sensitiveness of the retina. This is indicated in the figure above. 



The Ophthalmoscope. Prior to the invention of the Ophthalmo- 

 scope it was not possible to view the interior of the eye. The reason 

 of this is that when the interior is illuminated an image of the source 

 of illumination is formed in the retina, and the reflected light passing 

 from the illuminated area out again from the eye will be subject to the 

 refracting mechanism of the eye, 

 and form a small image in the 

 line of incidence of the source 

 of light. 



The Ophthalmoscope(Fig. 220) 

 is really a contrivance to enable 

 an observer to direct his vision 

 along the axis of the pencil of 

 light illuminating the subject's 

 eye, and thereby to enable him 

 to receive light reflected from 

 the retina of the subject, in other 

 words, to actually see the illumi- 

 nated retina. The instrument 

 consists essentially of a mirror, 

 in which is a central aperture. 

 The mirror is arranged so as 

 to reflect light from some source 



through the pupil into the in- Flo . 220. -Ophthalmoscopes. 



terior of the eye. The observer, 



looking through the central aperture, is able to view the illuminated 



posterior wall of the eye. 



Two methods are usually adopted of using the ophthalmoscope, one 

 being known as the direct, the other as the indirect. In the first case 

 there is obtained an erect view of a small area of the retina, magnified 



