THE MAGNIFYING POWER OF THE EYE. 



449 



fig. 218. 



ottier— the first the l-30th of an inch, and the secoiid the l-40th of 

 an inch in diameter j* let these be so dispoaed, that when the glass 

 slip, with a small object mount- 

 ed on its centre, is introduced 

 through the slits, the two aper- 

 tures and the object shall all oi i 

 respond, and be in one straig 1 1 

 line, while the slide is about 

 quarter of an inch behind tl 

 smaller opening, as in fig. 21 fi 



The image becomes more d 

 tinct when more than one ap< i 

 ture is used ; for the intensity ot 

 light by which it is illuminated 

 is thereby increased, being almost 

 in a direct ratio with the number of the openings which are employed. 



It is found that the magnifying power of the eye is limited by 

 the magnitude of the visual angle on the one hand, and by the in- 

 tensity of light on the other. If the visual angle be too large, the 

 rays are not sufficiently refracted by the humours of the eye to con- 

 verge to a focus and form an image on the retina ; and if too small, 

 the image is reduced to a mere point. The exact amount of diver- 

 gence of the rays, therefore, for any individual eye, lies somewhere be- 

 tween these two extremes. Again, however accurately adjusted the 

 visual angle may be to the refractive powers of the eye, if the light be 

 too strong the pupil becomes so contracted that only the innermost 

 rays are admitted; while if it be of small intensity, the object is so 

 dimly illuminated as to be scarcely visible. If, then, whilst a small 

 object is held very near to the eye, so as to ensure a rapid divergence 

 of the rays proceeding from it, the pupil can be dilated by the small 

 quantity of light which is used, and to which, like a photometer, it 

 ittmiiediately responds, so as to admit as large an angle as the lens 

 and humours of the eye are capable of refracting, at the same time 

 that the object is rendered distinctly visible, then, under such circum- 

 stances, we have arrived at the Utmost limit to the available magnify- 

 itig power of the eye." 



Well may we inquire with the immortal Nswtoti : " Was the eye 



* Sewing-needles are ordinarily sold in papers, numbered from 1 to 12, according 

 to their thickness. The diameters of apertures made with needles from the papers 

 marked Nos. 6, 7, 8, 9, and 10, when measured with the micrometer, are bqa&l tt> the 

 l'36th, the l-38th, the l-44th, the l-50th, and the l-70th of an inch. 



G O 



