NOTES AND MEMORANDA. 
339 
definition of an object (point for point) difficult of attainment. 3. The 
function of angular aperture of the objective. And he maintains that 
this latter part of the subject opens an entirely new aspect of the 
theory of the microscope which has been hitherto ignored. 
Whether a point or line of light, or shadow, can be seen, has abso- 
lutely nothing in common with the question whether that which is 
seen well (and wffiich is, of course, the optical expression of some fact) 
as an isolated object, can be equally well seen when in cb se appo- 
sition with a series of similar and equally minute objects. We cannot 
longer continue to interpret on the strength of an hypothesis which 
has been disproved. 
In the grosser cases of bad definition nobody pretends that dis- 
persion circles adequately represent the object under view, but neither 
could anyone charge upon this explanation (dispersion) that it proved 
or implied any ‘ limitation of visual power.’ 
And, in considering the subtle phenomena conditioned by the use 
of objectives constructed with large aperture, if it be proved that their 
optical function labours under various physical disabilities — among 
others, for instance, that rays of light may so interfere as to render 
void instead of presenting to the eye the actual structure from which 
they arise — this is surely no reason for confounding the fact that a 
structure cannot be seen under these given conditions, with the in- 
ference that an ‘ ultimate limit of vision ’ has been found for all other 
objects and under all other circumstances. It seems more natural to 
believe that minute particles in close juxtaposition shall, when viewed 
through a microscope, produce such interference of rays as to annul 
or distort the microscopic image ; although the same, or even much 
smaller particles, when isolated (not interfering with each other) may 
be distinguished. 
The flagellum was seen, not resolved —\i the term ‘ resolution’ has 
any meaning ! 
Abbe does assert that what has been, or can be resolved by a fg, 
or inch objective can be as clearly resolved by a jlg- <^1* iV immersion 
lens, because the additional amplification helps at farthest to enlarge 
the image, but it is an empty amplification. Take Dr. Woodward’s 
photographs of Diatomaceae with a Have the possessors of -g^^, 
TiV’ 23 " objectives seen anything more or newer than Mr. Stephenson’s 
Pleiirosigma drawing from i-inch immersion? 
Professor Abbe’s whole writings sufficiently prove that he has kept 
to the subject of geometric definition of images in the microscope, 
diffraction images, distribution of light in the microscopic image, 
illumination, &c. ; and these are so bound up together, and the theory 
of the microscope and the theory of illumination are so interconnected, 
that neither could be understood without the other. 
But the question of the minutest dot or line which can be seen is 
simply one of physiology ; thus, what is the least difference of light 
and shadow disposed in line or as isolated lights or shadows (white on 
dark or dark on white) which, thrown on the retina, can excite special 
nerve sensation, i. e. sight ? I exclude colour, because differences of 
colour are more perceptible than grades of more or less white light. 
