THE SENSE OF SIGHT 177 



capable of being thus contracted to one-sixtb of their original length. These 

 fibres, similar in their nature to those of which all muscular llesh consists, per- 

 tain, in the iris, to two systems having two different directions. The one system 

 consists of fibres which surround the pupil in circular lines concentric to its 

 margin ; the other of fibres perpendicular to the former, which radiate from the 

 margin of the iris to the margin of the pupil. In consequence of this arrange- 

 Dicut the operation of the two systems is necessarily as Ibllows : If the marginal 

 or circular fibres contract, the circle formed by them becomes smaller, and con- 

 sequently the pupil, whose margin is bordered by these circles, narrower; if 

 *'. ^ radial fibres contract, they draw the margin of the pupil outwards on every 

 ^iLle, and therefore widen the pupil. Now it is clear that the narrower the pupil 

 the fewer will be the rays of light which can penetrate within the eye to the 

 retina. Ihe use of a change m the diameter of the pupil is this : if a great deal 

 of light is thrown by very bright objects upon the eye, a part thereof is cut off 

 by the narrowing of the pupil, in order that the sensitive retina may not be un- 

 duly excited and dazzled ; if we look, on the other hand, at obscure objects, the 

 • pupil is widened in order to admit as many rays as possible into the eye, to 

 communicate the strongest excitatitm to the retina and produce the m >st vivid 

 sensation possible. It is a circumstance of the greatest interest that this regu- 

 lation of the intensity of the light arises not through an act of the will, but with 

 invariable certainty and proportionality through a reflex mechanism. It is the 

 light itself which sets the mechanism in motion with a force corresponding to its 

 own strength ; the impression produced by the liglit on the retina and propa- 

 gated by the optic nerve to the brain calls forth there an activity which, in a 

 reflex manner, is communicated to the muscles of the iris. 



Having thus given a general idea of the structure of the eye, we proceed to 

 explain the formation of a small inverted image of the external object on the 

 retina. lu the above figure a h represents an arrow placed before the eye, 

 which arrow we will imagine to be composed of a number of single luminous 

 points arranged one after the other. Such a luminous point is a at one end and 

 b at the other end of the arrow. Every such point emits rays of light in all 

 directions, so that the space pervaded by these rays represents a globular sphere, 

 of which the luminous point is the centre. If the eye be within this sphere, a 

 small segment of it, that is to say, a small pencil of rays, penetrates into the 

 eye, namely, those rays which fall within the circle of the pupil, while the rest 

 are intercepted by the opaque parts of the eye. In the figure the diverging, 

 dotted lines proceeding from a, and those formed of small strokes proceeding 

 from h, show the limits of the pencil of rays (in transverse section) which are 

 capable of entering through the pupil into the eye. Now, we have already men- 

 tioned that the crystalline lens and cornea together refract the incident rays 

 from their course in such a manner that they cause the rays which, in front of 

 them, were divergent, to approach one another in their rear, so as to unite again 

 at a point situated behind the lens. How and by what laws this refraction takes 

 place we cannot stop to explain. For simplification let it be supposed that the 

 refraction of the rays is only produced by the lens ; the figure will then show 

 the rays proceeding from a as united behind the lens at a', and those proceeding 

 from h at h' ; a' is therefore the image of the point a, as // is of Z». Between d 

 and h' lie the images of all the luminous points of the object between a and h ; 



a' h' then is the image of a h. The figure shows further, first, that the 



image is smaller than the object; and secondly, that it is inverted. In the ex- 

 ternal arrow, a lies to the left, the corresponding image a' to the right ; h lies to 

 the right, h' to the left; the relative- position of all the intermediate points be- 

 tween a' and V will therefore be the same. In this way there is formed on the 

 background of the eye a small inverted image of every external object from 

 which rays pass into that organ, as a similar image is traced on the blank tablet 

 of the camera obscura The experimental verification of these facts is not dif- 

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