605 



to the screen) which joins the foci. But the deflections of the 

 shadow in the two sorts of rays are in adverse directions, so that if 

 the axis of a single pencil were moved across the eye, whilst always 

 kept parallel to itself, the shadows of all the objects lying in advance 

 of the focus would travel in one direction, whilst those of objects 

 lying behind the focus would travel in the opposite, the rate of 

 movement being in both instances greater for objects nearer the focus. 

 Also, whether with a couple of pencils or a single one in movement, 

 for a given difference in ocular depth between two objects, the 

 difference in parallactic deviation is greater as the two objects are 

 nearer the foci or focus. Besides, generally, the picture of the 

 contents of the eye, as shown by the convergent portion, is inverted 

 in the divergent. 



The above proportions are turned into equations which indicate, as 

 the terms alter their values, every observed variation in the sizes of 

 the shadows, parallactic deviations, inversions of figure, place, and 

 movements whether of the ocular bodies with respect to the pencils, 

 or of the foci and axes of the pencils with respect to them. Thus, 

 in many modes, which are explained, we may at pleasure by mere 

 inspection observe the structural and relative positions of the bodies 

 in question, and can so manage to evade the effects of ocular refrac- 

 tions, as to render the proportions above stated available for calcu- 

 lating the sizes of the bodies, and their distances from the cornea, 

 iris, faces of the crystalline lens, retina, or from any one of themselves 

 or can even measure their depths in the eye, almost without cal- 

 culation, and free from any that involve the consideration of the 

 optical qualities of the organ. 



Inflective phenomena are alike in the convergent and divergent 

 rays, but refractive differ, and afford us a useful means of detecting 

 the nature of an object. Inflective coloration is too subordinate to 

 the ocular chromatic dispersion to deserve particular notice. Dr. T. 

 Young explains how narrow straight objects, viewed through a 

 puncture, are by the influence of ocular refractions made to appear 

 curved, unless they are seen as diameters of the projected image of 

 the pupillary opening. It is appended, that if they are made to 

 encroach, in a like way, laterally upon a divergent pencil, they appear 

 not, as in the case mentioned, concave, but convex towards the 

 centre of the pupil's image. 



2x2 



