594 BARON VON HELMHOLTZ. 



of the mode in which the eye instantly and involuntarily adapts itself 

 for various distances, near and far. It was reserved for Helmholtz 

 and his collaborator, Cramer, to solve this problem, by proving that 

 an adaptive accommodation is effected by an increase of convexity of 

 the crystalline lens, and principally of its anterior surface, through the 

 agency of the ciliary muscle." The demonstration of these facts, by 

 means of oblique illumination, was in this wise. 



In a darkened room, a lamp fiame, placed near the eye to be ob- 

 served at an angle of about 30° from its visual axis, gives, while the 

 eye looks at a distance, to an observer placed at about the same angle 

 at the opposite side of the visual line, three reflected images of the 

 flame: one erect, from the cornea; another, also erect, from the ante- 

 rior capsule of the crystalline lens; and a third, an inverted and 

 smaller image, from the posterior capsule of the crystalline. If now 

 the observed eye, instead of looking at a distance, accommodates itself 

 for a near object, say at twelve inches, the reflected image from the 

 anterior surface of the lens appears lessened in size and approaches 

 the corneal reflection, — a change which could result only from an 

 increase of convexity of the anterior surface of the crystalline. The 

 inverted image from the posterior capsule of the lens remains prac- 

 tically unchanged in size or position, proving that but slight, if any, 

 change of curvature occurs at this posterior surface of the lens during 

 accommodation for near objects. 



Of all proof, we deem ocular evidence the most conclusive. Thanks 

 to Professor von Helmholtz, we possess, in his supreme inquisitor, 

 the opthalmoscope, an interpreter not only capable of revealing and 

 explaining hitherto unknown physiological laws of normal vision, but 

 which also affords clear and positive disclosures of previously unsus- 

 pected morbid processes, which, if unarrested, may imperil the most 

 precious of our senses, and our chiefest means of instruction and of 

 enjoyment. More than by any other faculty we live, and move, and 

 learn, and enjoy by that of vision. To our eyes we owe almost every- 

 thing we have, and are, and hope to become. 



Were the revelations of the ophthalmoscope as to pathological and 

 physiological conditions limited to such as affect the eye itself, they 

 would still have an infinite value. But these disclosures have a still 

 wider range, showing, through changes discoverable by its means in 

 the retina and other internal structures of the eye, the advent of yet 

 graver disease in other remote and most important parts of the body, 

 — as, for instance, in the brain or the kidneys, — before the presence 

 of any morbid tendency has been suspected. 



