546 Royal Society : — 



luminous objects, upon a membrane in special relation with the 

 brain, but involve many adjuvant structures ; and thus it happens 

 that thej' reveal to us a number of adventitious phenomena — 

 spectres as we may call them, whether caused by light at the 

 ])arts that cover the eyeballs, or within them, or by any stimulus 

 whatever affecting the special nervous tract. These must be elimi- 

 nated, if we would avoid the risk of ascribing effects begotten 

 by subordinate parts to more integral portions of the apparatus. 

 Finally, they may be rendered serviceable for the solution of certain 

 important points of ocular structure and function. Under the im- 

 ])i"ession that so diversified a subject has not yet received all the 

 elucidation of which it is susceptible, another methodical attempt to 

 investigate it will be made in this memoir. 



When light is an agent in the production of the spectral pheno- 

 mena, they arise from certain rays being blocked from their course, 

 at some obstacle thej^ encounter, or turned aside by refraction, reflex- 

 ion or inflection. And to vaske precise observations upon them, we 

 must use fine pencils of rays which do not return to foci upon the 

 retina. In order to estimate the relative and actual sizes, localities, 

 and characters, of corpuscles whose shadows or images are projected 

 upon the retina, pencils of light which are first convergent, and 

 therefrom, by passing through foci, divergent (such as may be 

 conveniently obtained from a small disc of light at a sufficient 

 distance from the eye, when viewed through a lens of an inch focal 

 length), are mainly employed, the foci being carried from before the 

 eye to various depths in its interior*. If we neglect ocular refractions, 

 whether a body fall in the convergent or divergent portions, the 

 length of its shadow will be to its own, as their respective distances 

 from the focus. With a couple of such pencils, whether a body fall 

 in the convergent or divergent portions, the distance between its pair 

 of shadows is to that between the foci, as that of the object from 

 the shadow screen (retina) to that of the object from the line (parallel 

 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 



* In the Allgem. Encyklop. der Pliysik, s. 166 (1856), in an able article on 

 " Entoptics," Helmholtz states, that " the more decided entopticai ' methods ' 

 were eatahhshed first by Listing and Brewster (1845), who were followed still 

 later by Donders (1846-51)." The present wTiter refers to a paper of his own, 

 published early in 1845, which substantially gives the methods alluded to ; which 

 are all modifications of one idea, that of obtaining a greater parallactic deviation 

 of shadows for objects further from the retina, by means of two pencils of rays 

 diverging from points in front of the eye, or by one moved across the optic axis. 

 Now we can by this device only get marked differences in parallax for small dif- 

 ferences in ocular depth for objects very near to the points of divergence, that 

 is, near the surface of the eye; whereas the plan now proposed not only generally 

 secures this end in a notable degree, but by placing foci between any two objects, 

 causes their shadows to be deflected in opposite directions, and the more con- 

 siderably as they are nearer together ; besides supplying, it is believed, a variety of 

 aids in entopticai researches. 



