262 PRACTICAL PHYSIOLOGY 



mirror is the direct source of illumination of the subject's eye, 

 and this image moves to the right when the mirror is tilted to the 

 right, and in accordance with the inversion of the image on the retina 

 the illuminated area will seem to pass to the left. The general impres- 

 sion that one obtains of the result of tilting the mirror on the emme- 

 tropic eye is that the illumination suddenly disappears. With the 

 hypermetropic eye the illuminated area is more distinct, as a large part 

 of it can now be seen, and the passing of this area to the right or left 

 inversely to the tilting of the mirror to the left or right is clearly 

 visible. In the case of myopia the observer must be beyond the far 

 point of the eye and then will see an inverted image of the illuminate 

 area. As the result the apparent illuminated area will be an inversion 

 of the actual area. When therefore the mirror is tilted and the image 

 of the lamp passes across from right to left, the apparent movement will 

 be from left to right, so that the movement of the light on the retina 

 appears to be the same as the tilt of mirror. A small amount of 

 myopia cannot be made out by this method. 



EXPERIMENT. If subjects possessing the defects of myopia and 

 hypermetropia cannot be obtained, using the ophthalmoscopic mirror as 

 directed above, observe the movement of the light on the retinal screen 

 in Kiihne's artificial eye adapted for these defects. Compare the actual 

 movement of the light on the screen with the apparent movement when 

 observing in front of the eye as above. 



2. Imperfections of the Refracting Media, Entoptic Phenomena. (a) 

 Certain bright, cloudy appearances may be seen, which disappear after 

 blinking the eyelids. Wavy lines or speckled patches may appear after 

 rubbing the eyes. These are all due to the condition of the corneal 

 surface, and have been more properly called ' pseudentoptic ' phenomena. 



(b) Dark specks or irregularly stellate figures may be seen, depending 

 upon imperfections in the lens or its capsule. 



(c) Muscae Volitantes. These appear as moniliform threads, clusters of 

 bright or dark circles, and are referable to imperfections in the vitreous. 



EXPERIMENT. Place a card which is pierced by a pinhole a little 

 more than a centimetre from the eye (i.e. in the position of the principal 

 anterior focus of the ' reduced ' eye). Look at an evenly but brightly 

 illuminated surface beyond, as a sheet of thin white paper held in front 

 of a lamp. The rays of light falling on the retina are now approxi- 

 mately parallel, and any shadows that form in consequence of imper- 

 fections in the refracting media are rendered more distinct. Notice 

 any of such shadows that may be received by blinking, due to im- 

 perfections in the cornea or any comparatively fixed figure due to 

 imperfections in the crystalline lens. These may be practically absent. 



