;jg2 SIR DAVID BREWSTER ON THE OPTICAL PHENOMENA, NATURE, 



will have their double images very distant : those in the middle of it will have 

 their double images much nearer : those near the retina will have their two images 

 close or perhaps overlapping each other ; while any object on the retina itself, any 

 black spot arising from defective sensibility, will have only one image, as it were. 

 Now, if we measure the distance of the two sources of light from each other, and 

 also their distance from the centre of visible direction, when the two images of 

 the filaments, &c., are just in contact, we may determine the size of the filament 

 and its exact position, as well as its distance from the retina. In making this 

 experiment, I first found that the angle of apparent magnitude of the shadoAv 

 of the filament ABC was eight minutes, and consequently [that it subtended 

 this angle at the centre of visible direction.* Now, if we take the radius of the 

 retina as 0.524 of an inch, the diameter of the shadow of the filament will be 

 0.0122, or g|gth of an inch, and its distance from the retina 0.018, or g^th part of 

 an inch. 



When we use a small aperture alone for producing a divergent pencil, the 

 centre of divergency must necessarily be without the eyeball ; but we may throw 

 the centre of divergency within the eyeball, and place it at any distance from the 

 retina, by using a lens of the proper focus. If we wish to place this centre near 

 the retina, a lens of considerable focal length must be used, and as the light col- 

 lected by it will be powerful, it will extinguish all the smaller filaments and mi- 

 nute spheres, and allow only the larger Muscte to be seen. We must therefore 

 reduce its aperture by looking through a pin-hole or other minute opening. When 

 we wish to have a clear field of view for examining the larger Muscte, we may ex- 

 tinguish all the smaller ones by increasing the luminosity of the field. If we 

 wish to study the filaments or Muscce that may be placed about the middle of the 

 vitreous humour, we must use a lens of such an aperture as will obliterate all 

 those more remote from the retina. 



It is very obvious, from the preceding observations that objectsplaced within the 

 eyeball are not seen, as Dr PORTERFIELD believes, by rays which pass THROUGH dense 

 particles having suffered a greater refraction than those which pass BY them. A fibre 

 or particle of glass of nearly the same refractive power as the vitreous humour will 

 be seen distinctly by means of its image formed on the retina by diffracted pencils. 

 If the light is not sufficiently divergent, or is too intense to produce and exhibit 

 the diffracted image, the object will be invisible, unless it be of such a size, and so 

 near the retina, as to shew itself by its ordinary shadow. But in whatever way 

 the image of the object is formed, the mind takes cognizance of it, or gives it an 

 external locality, by means of the same law of visible direction which regulates 

 the vision of objects placed without the eyeball. 



* This may be done by projecting it upon a luminous surface, and marking its apparent size ; or by 

 comparing it with the images of objects of known dimensions seen with a fine microscope. 





