* ophthalmoscope. 329 



angle of incidence is the same as the angle of reflection. In the 

 hollow fundus of the eye, however, the light entering through 

 the narrow pupil, strikes the fundus at a point which is hidden 

 from the observer, behind the iris, and being reflected by the 

 concave fundus, in exactly the same line along which it entered, 

 it remains invisible. To illuminate the fundus of the eye, for 

 the observer, his line of vision must be made exactly the .same as 

 that in which the pencil of light enters the fundus. This is best 

 effected by reflecting the light into the eye by the aid of a small 

 plane or concave mirror having a hole in the center through 

 which the observer looks into the pupil. The concave mirror 

 gives the .stronger illumination, but tlie plane article is more 

 easily manipulated and tends to cause less active contractions of 

 the pupil. This is the simplest form of ophthalmoscope. For 

 careful examination of the fundus of the eye, it is best to have 

 the subject in a dark chamber, with a single large flame of an oil 

 lamp or gas (electric light with an obscure globe may answer). 

 The light is held behind and on the same side as the eye to be 

 examined, at the level of the e3-e and the perforated mirror and 

 the eye of the observer are kept from 10 to 20 inches in front of 

 the eye and also at the same level. For the horse or ox under 

 favorable conditions in a stall, the light of day coming from a fan- 

 sash over the door may serve the purpose. Nicholas assures us 

 that it may be accomplished even under the shadow of a shed or a 

 tree. In such a case it is better not to have too much glare of 

 light as the reflection from cornea and lens may prevent accurate 

 observation. A somewhat cloudy day may therefore prove ad- 

 vantageous. 



In focusing the reflected light on the cornea, and then on the 

 pupil and lens, any opacities in these will be shown as a grayish 

 nebular reflection or a denser white according to their degree of 

 opacity. The opacities in the cornea or aqueous, in front 

 of the axis of vision in the lens move in the same direction and 

 to the same degree as the eye rolls, while opacities on the pos- 

 terior capsule or in the vitreous, move in a direction opposite to 

 the motions of the eye, and to a degree corresponding to their 

 distance back of the lens. Thus if the eye looks downward such 

 opacities move upward ; if it looks upward they move downward ; 

 if it looks inward they move outward ; and if it looks outward 

 they move inward. 



