and Locality o/"Miisca2 volitantes. 7 



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 itbelf, 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 experi- 

 ment, 1 first found that the angle of apparent magnitude of 

 the shadow of the filament ABC was eight minutes, and con- 

 sequently that it subtended this angle at the centre of visible 

 direction *. Now, if we take the radius of the retina as 0"52't 

 of an inch, the diameter of the shadow of the filament will be 

 00 J 22, or Tjlijth of an inch, and its distance from die retina 

 0*0] 8, or ^ijlh part of an inch. 



When we use a small aperture alone for producing a diver- 

 gent pencil, the centre of divergency must necessarily be with- 

 out 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 collected by it will be powerful, it 

 will extinguish all the smaller filaments and minute spheres, 

 and allow only the larger Muscat to be seen. We must there- 

 fore reduce its a|)erture by looking through a |)in-hole or other 

 minute ojiening. When we wish to have a clear field of view 

 for examining the lai-ger Musae, we may extinguish all the 

 smaller ones by increasmg the luminosity of the field. If we 

 wish to study the filaments or Mtiscce that may be placed about 

 the middle of tiie 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 

 objects placed within the eyeball are not seen, as Dr. Porter- 

 field believes, by rays tohic/i pass through dense particles 

 having suffered a greater refraction than those vohich pass by 

 them. A fibre or particle of glass of nearly the same refrac- 

 tive power as the vitreous humour will be seen distinctly by 

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

 II' the light is not sufficiently divergent, or is too intense to 

 produce and exhibit the diffracted image, the object will be 



• This maj' be done by projecting it upon a luminous siirPace, and mark- 

 ing its apparent size; or by comparing it with the images of objects of 

 known dimensions seen with a fine microscope. 



