596 THE MICKOSCOPE. 



"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 intensity 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 converge 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 

 divergence of the rays, therefore, for any individual eye, 

 lies somewhere between these two extremes. Again, how- 

 ever 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 imme- 

 diately 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 circumstances, we have arrived at the 

 utmost limit to the available magnifying power of the 

 eye." 



Indeed, the improvement of the microscope, as well as 

 other optical instruments, gradually followed after the 

 wondrous perfection of the Dioptric apparatus of the 

 human eye, became unfolded to the scientific mind. " Be- 

 fore the days of Sir Isaac Newton, men knew that they 

 saw by means of the eye, and that the eye was constructed 

 upon optical principles; but the reason of its peculiar 

 conformation they knew not, because they were ignorant of 

 the different refrangibility of light. When his discoveries 

 taught this truth, it was found to have been acted upon, 

 and, consequently, known by the Being who created the 

 eye. Still, our knowledge was imperfect, and it was 

 reserved for Mr. Dolland to discover another law of nature 

 the different dispersive powers of different substances 

 which so enabled him to compound an object-glass that 

 more effectually corrected the various refrangibility of the 

 rays. It was now observed that this truth must also have 



