SCHEINER'S EXPERIMENT. 



753 



necessary for the image reflected from the anterior surface of the lens to change its place during 

 accommodation, is less than that required for subjective accommodation (Aubert and 

 Angclncci). 



8. Line of Accommodation. When the eye is placed in a certain position during accommo- 

 dation, we may see not one point alone distinctly, but a whole series of points behind each 

 other. Czermak called the line in which these points lie the line of accommodation. The more 

 the eye is accommodated for a distant object, the longer does this line become. All objects 

 placed at a greater distance from the eye than 60 to 70 metres appear equally distinct to the 

 eye. The line becomes shorter the more we accommodate for a near object i.e., when we 

 accommodate as much as possible for a near object, a second point can only be seen indistinctly 

 at a short distance behind the object looked at. 



9. The nerves concerned in the mechanism of accommodation are referred to under Ocnlo- 

 motoriiLS ( 345, and again in 704). 



Scheiner's Experiment. The experiment which bears the name of Scheiner 

 (1619) serves to illustrate the refractive action of the lens during accommodation 

 for a near object, as well as for a distant object. Make two small pin-holes (S, d) 

 in a piece of cardboard (fig. 537, K, Kj), the holes being nearer to each other than 

 the diameter of the pupil. On looking 

 through these holes, S, d, at two needles 

 (p and r) placed behind each other, then 

 on accommodating for the near needle (p), 

 the far needle (r) becomes double and in- 

 verted. On accommodating for the near 

 needle (p), of course the rays proceeding 

 from it fall upon the retina at the focus 

 (Pi) > while the rays coming from the far 

 needle (r) have already united and crossed 

 in the vitreous humour, whence they di- 

 verge more and more and form two pictures 

 ( r /> r ) on the retina. If the right hole in 

 the cardboard (d) be closed, the left picture 

 on the retina (r ti ) of the double images 

 of the far needle disappears. An analo- 

 gous result is obtained on accommodating | 

 for the far needle (R). The near needle 

 (P) gives a double image (P /5 PJ, because 

 the rays from it have not yet come to a 

 focus. On closing the right hole (d i ) 1 the 

 right double image (P,) disappears (Porter- 

 field). When the eye of the observer is 

 accommodated for the near needle, on 

 closing one aperture the double image of 

 the distant point disappears on that side ; 

 but if the eye is accommodated for the distant needle, on closing one hole the 

 crossed image of the near needle disappears. 



K, 



Fig. 537. 

 Schemer's experiment. 



388. REFRACTIVE POWER OF THE EYE ANOMALIES OF RE- 

 FRACTION. The limits of distinct vision vary very greatly in different eyes. 

 We distinguish the far point [p.r., punctum remotum] and the near point [p.p., 

 punctum proximnm] ; the former indicates the distance to which an object may 

 be removed from the eye, and may still be seen distinctly ; the latter, the distance 

 to which any object may be brought to the eye, and may still be seen distinctly. 

 The distance between these two points is called the range of accommodatioE. 

 The types of eyeball are characterised as follows 



1. The normal or emmetropic eye is so arranged when at rest that parallel rays 

 (fig. 538, r, r) coming from the most distant objects can be focussed on the retina 



3b 



