HERMANN VON HELMHOLTZ 



and at the next the distant object. Again, if we 

 persistently look at objects close to the eye, say 

 within 6 inches, there is a sense of effort and the 

 eye becomes fatigued. The range within which the 

 accommodating mechanism works is, in the normal 

 eye, from 65 metres (about 70 yards), the so-called 

 punctum remotum^ to -^th metre (20 centimetres, say 

 10 inches), the punctum proximum. Beyond 65 metres 

 rays emanating from an object may be practically 

 regarded as parallel, and they will be focussed on the 

 retina without effort ; from 65 metres to ^th metre 

 the accommodating mechanism comes into play so as 

 still to bring the more divergent rays to a focus on 

 the retina, and thus secure distinct vision ; and 

 lastly, within a distance of 4th metre (except in juvenile 

 life) the accommodating mechanism ceases to act. the 

 rays that enter the pupil are now too divergent to be 

 focussed on the retina, and there is blurred vision. 

 If we cut off the more divergent rays by looking at 

 the near object (say 2 inches from the eye) through 

 a hole in a card, then we can see the object dis- 

 tinctly as we now use the central pencils of rays 

 which may be slightly divergent or nearly parallel. 

 The pupil, the diameter of which is lessened by con- 

 traction of the iris, serves the same purpose. Con- 

 sequently, when we look at a near object the pupil 

 contracts. The question arises, how is this wonderful 

 mechanism carried out ? 



If we hold a lighted candle in front, and a little to 

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