PROCEEDINGS OF THE SOCIETY. 
389 
The President said there was no attempt made to try the extreme 
distance at which the hairs could be seen, all the persons were re- 
quired to do was to state a distance at which they could be distinctly 
seen. In one case the person did not know the position of the hairs ; 
it was one of his men who had just come up from the village and did 
not know what they were about. In another case a person saw the 
horizontal hair, but failed to see the vertical one ; and there was no 
endeavour made to ascertain who could see the objects at the greatest 
distance. One suggestion bearing upon the subject he thought might 
be a point for consideration, and that was that seeing the image of an 
object of 1 millionth of an inch in diameter might not be the same as 
seeing an object of that diameter. He had hoped to have seen Professor 
Stokes this evening, but he was engaged at Cambridge. 
Mr. Crisp said that as Professor Stokes had been referred to, it 
might not be without interest to read an extract from a letter refer- 
ring to the subject, addressed by that gentleman to Mr. Mayall some 
time since : — 
“ As to Fraunhofer’s formula, it is that which applies to the diffrac- 
tion spectra formed by fine equidistant lines, and to that alone. The 
whole of the circumstances are so different from those of the illumi- 
nation and viewing of microscopic objects, that we have no right, I think, 
to draw any conclusion against the visibility of very fine objects, even 
though they be Nobert’s lines, and therefore of the same nature as the 
object to which Fraunhofer’s formula refers. In the Fraunhofer spectra 
the object is illuminated by nearly parallel rays, and the instrument 
is focussed for the distant slit, and the angular aperture is some 3° or 
4° or less. When Nobert’s lines are used as a test, the object is illu- 
minated by strongly condensed light, the instrument is focussed for 
the lines themselves, and the angular aperture of the microscope 
objective is very much greater.” 
He also read the following note from Mr. Mayall, which accom- 
panied it : — 
“ In Arago’s ‘ Astronomy ’ there are some curious remarks made on 
Photometry, which I think bear strongly on the question of the limit 
of visibility. Briefly this is the result at which Arago arrived ; — 
If two adjacent points on a surface be unequally illuminated, the eye 
is capable of perceiving the difference when it is greater than ; a 
less difference than (or thereabouts) cannot be seen by the eye, 
and the two points will appear equally illuminated, and therefore not 
discernible as separated. 
“ Applying this experiment to the microscope : suppose we have an 
anatomical preparation, unequally transparent, so that the most trans- 
parent part allows 61 rays to pass through, whereas the adjacent part 
allows only 60 rays to pass through ; the eye still perceives the 
difference between them. But this is the limit (or thereabouts), 
otherwise the eye sees only a uniform surface. 
“ This difference of that follows from Arago’s experiments as the 
limit of perception by the eye, is not an absolute quantity, but will 
depend somewhat on the sensitiveness in individual eyes. 
“ Again, too, it will depend on the intensity of the light in which 
