Intelligence and Miscellaneous Articles, 317 



always real, between the length of the principal and of the conjugate 

 focus of a point situated at two inches from the lens are only 



About 1 millim. in the ox and sheep, \ millim. in the pig, -J- to £ 

 millim. in the human subject. 



Hence, to pass from distant vision to the vision of objects of 2 

 inches' distance, the eye needs an apparatus which, in the case of 

 man, makes the interior conjugate focus traverse a distance of 2*5 to 

 3 millims. between parallelism of the incident rays and the diver- 

 gence corresponding to 2 inches. 



II. The optic centre of the entire eye, and even that of the crys- 

 talline isolated in the air, are behind the posterior face of the crySf 

 talline. In the eye of the albino rabbit, the only one in which the 

 position can be accurately determined, the optic centre is exactly in 

 the centre of figure and of motion of the globe. 



A decisive physiological experiment shows the exactitude of this 

 coincidence in the human eye at the time of the regular exercise of 

 sight. The pupil is dilated by means of atropine ; then by means 

 of a fixed binocular ophthalmoscope, the sharp image of the flame of 

 a candle is observed on the choroid by Knapp's method. The sub- 

 ject retaining his head quite fixed, his eye is caused to make regular 

 and slow motions from one angle of the orbit to the other. 



During these movements the image neither varies in magnitude 

 nor in position, as can be established by a micrometer placed in the 

 very point occupied by the inverted image. It follows evidently 

 from this, that the optic centre coincides with the centre of motion 

 of the globe. 



This experiment further gives rise to the following accessory ob- 

 servation : if the narcotic used is weak enough not to have extin- 

 guished or materially impeded the accommodation, and the subject is 

 led to make an effort directing his attention to a distant point in the 

 same direction, as in Crammer's experiment, the image of the lamp, 

 at first quite definite and sharp in its contours, suddenly becomes 

 extended, confused, and badly defined. The change in the dioptric 

 state during the adaptation of the eye is thus demonstrated once 

 more. 



The situation of the optic centre of the eye in the very centre of 

 its rotation, behind and beyond the crystalline, evidently renders 

 illusory the skilful calculations on which Listing bases the construc- 

 tion of his schematic eye — illusory, not from the mathematical point 

 of view, but from the application to complex organic elements of 

 the beautiful formulae established by Gauss for homogeneous optical 

 elements. 



III. Referring the elements of the schematic eye to the real posi- 

 tion of the optic centre, we shall represent the schematic eye in one 

 or other of the two following manners, according as we want to 

 approach more or less to the arrangements of the real eye. 



The most simple combination would be a sphere transparent in 

 front, terminating a homogeneous refringent medium, whose index 

 of refraction is 2, and whose diameter is 23 millims. 



The focus of such a system is at the extremity of its diameter, and 

 the optic centre in the centre of the sphere. 



