Diagrams and other drawings accompany these and other parts of 

 the essay. The foregoing principles are henceforth applied to the 

 actual exploration contemplated, in order, as follows : 



SECTION II. Apparitions from Eyelashes, Eyelids and 

 Conjunctival Fluids. 



These phenomena are treated with an effort at greater pre- 

 cision than in previously existing accounts of them. Little bars 

 of fluid along the margins of the lids are shown to occasion the long 

 beams of light, which issue from flames regarded " with winking 

 eyes," by their annulling the refractions of the cornea. These beams 

 have been ascribed to reflection at the edges of the lids, but reflection 

 only yields a very pale beam which can be distinguished easily from 

 the other. 



SECTION III. Apparitions from Iris and Crystalline Lens : with 

 Corollaries. 



The margin of the iris, opaque and transparent bodies, and the 

 structural stellate figure, in the crystalline lens are placed methodi- 

 cally in the order in which they lie in the depths of the eye, and 

 the especial manifestations which they severally yield, explained. 

 The combined effects of ocular chromatic aberration, inflection 

 at the edge of the iris, and the limbs of the stellate figure when we 

 look at thin objects, or black and white lines, especially if curved, 

 render some singular illusions, which are dissected. 



The method by the two sorts of pencils may be applied to test 

 the recent doctrine advanced by Stellweg, that the iris so lies on 

 the face of the crystalline lens, that there is no posterior chamber 

 in the aqueous humour, and will probably be found to disprove it. 



A calculation is entered into to show that unless Dr. T. Young 

 in estimating that the accommodation of the eye to focal distance 

 by means of an alteration in the length of the optic diameter, would 

 require a faculty of doing this to the extent of ^th of the whole, 

 taken when vision is suited to parallel rays, exceeds the truth by 

 many times ; it must be easy to detect, by the parallax of the lenticular 

 corpuscles in a couple of pencils whose foci rest near them, how and 

 where the change is effected. And then an argument is drawn, that 



