266 PRACTICAL PHYSIOLOGY 



about thirteen times ; in the second case a less magnified and inverted 

 view is obtained of a larger area of the retina. 



The Direct Method. The source of light is placed at the side of the 

 head of the subject, so that no light falls directly on the cornea. The 

 mirror, which is somewhat strongly concave, is held a few inches from 

 the subject's eye, and is so tilted that light is directed into the pupil. 

 The observer uses his left eye to examine the subject's left eye, and 

 similarly his right eye for the subject's right eye. By bringing the 

 light very close to the mirror, and this again close to the eye, the 

 subject will not be able to accommodate for the image of the source 

 of light, and consequently a dispersion circle of light will fall 

 upon the retina. If the observer look through the aperture and 

 the subject's eye be emmetropic he will obtain a clear view of the 

 details of the retina. The reflected light from the subject's retina 

 will issue as parallel rays and thus be in an appropriate state to 

 impinge on the observer's cornea without requiring him to make 

 any effort of accommodation. 



The Indirect Method. In this case a somewhat larger, but less con 

 cave or a plane mirror is used. The mirror is held at a distance of 

 about eighteen inches, and if the accommodating power of the subject is 

 intact his eye will accommodate for the source of light or its image 

 formed by the mirror. An inverted image of the illuminated area of 

 the retina will be formed at a certain distance behind the mirror. If 

 the rays issuing from the eye be intercepted by a rather strong convex 

 lens held close to the cornea a new image will be formed, smaller and 

 more brilliant but still inverted. The observer then looks through the 

 aperture of the mirror, and holding a lens as above against the cornea 

 obtains a clear view of a considerable portion of the illuminated retina. 



Ophthalmoscopes are generally supplied with a revolving disc of lens 

 of different strengths, which are used to correct any error of refraction 

 in the subject's or observer's eyes. 



It is frequently a matter of difficulty to obtain a clear view of the 

 back of the eye or fundus in the subject unless some drug previously 

 has been applied which causes dilation of the pupil. For practice in the 

 use of the ophthalmoscope, an albino rabbit, the eye of which has been 

 treated with atropin, can be advantageously substituted for the human 

 subject ; or artificial eyes, such as Frost's or Perrin's artificial eyes, may 

 be used. In absence of these, the ocular of a microscope furnishes the 

 material for the construction of an artificial eye. If the lower lens 

 of this be removed and a disc painted to represent the fundus be 

 inserted and blocked behind, an artificial eye is obtained which can 

 be used with advantage. 



