the Index of Refraction of the Lens. 429 



the preceding Tables, numbered 1, 2, 3. The other three dia- 

 phragms having threads stretched across them, were placed at 

 distances corresponding to the diameters of the holes. Bringing 

 the brass plate up to the pupil of the eye so that all the holes 

 could be seen through at once, it was satisfactorily determined 

 that each thread could be seen distinctly at the same moment 

 with a church at a considerable distance, not by successive fitful 

 glances, but all steadily and sharply discernible at the self-same 

 moment, — none, besides, seen distinctly (as was subsequently 

 ascertained), except through its own hole and at its own parti- 

 cular distanee*. 



17. It will be gathered from the preceding remarks, that there 

 are questions relating to the phenomena of vision open to still 

 further investigation — that facts are wanted which can only be 

 obtained by most delicate experiments and carefully conducted 

 observations. It may, in truth, be said that nothing whatever 

 has been done to make us better acquainted than we were half 

 a century ago with the refractive densities of the humours, — one 

 value at the present moment being only guessed at, assumed, to 

 satisfy previously adopted notions f. The law of decrease of the 



* It follows as a corollary from this, that in the case of persons with 

 presbyopic eyes, if a horizontal row of some half-dozen holes f- millim. in 

 diameter be pierced in a narrow sheet of brass at distances of If to 2 mil- 

 lims. from each other, and a similar set in the same line for the other eye, 

 the lower edge of the little sheet of brass being curved to suit the contour 

 of the face, bringing this close up to the eyes, vision at ordinary distance 

 is so restored that the experimenter can read with tolerable facility ; and 

 that this is not due, as might be argued, to excluding all but parallel rays, 

 is readily determined by using holes of sizes suited to other distances. 



When the pupil of the presbyopic eye is diminished by inserting beneath 

 the lid paper prepared with the Obi bean {Phytostigma venenosa), vision at 

 short distance is restored. The effect does not last long, and is somewhat 

 impeded by the lachrymation which accompanies the use of the paper; but 

 of the fact there can be no doubt. 



f " It is not my intention to subject anew to criticism the long series of 

 incorrect views on the subject. I am not writing a history of errors. "We 

 now know what change the dioptric system undergoes in accommodation; 

 and the source of this knowledge alone can be here sketched in its leading 

 features. The change consists in an alteration of form of the lens ; above 

 all, its anterior surface becomes more convex and approaches to the cornea." 

 —Y. C. Bonders, ' On Accommodation and Refraction of the Eye,' Trans- 

 lation, p. 10. 



" It was not until after direct proofs (within reach of every one's obser- 

 vation and comprehension) of the change of form of the lens had been 

 brought forward by others that Helmholtz placed the able investigation of 

 Thomas Young in its proper light." — Op. cit. p. II. 



"The subject is one of importance; for it is only by taking this higher 

 index of refraction into account that we get rid of the paradoxical result 

 that, in a well-formed eye, parallel incident rays should be brought to a 

 focus behind the retina. The coefficient of refraction is no?/ fixed by Lis- 

 ting at \'Ahb." — Op. cit.ip. 39. 



