Photographs of Noherfs 19th Band. By H. C. Sorhy. 7 
* others to do so. I will therefor.e state that the photograph was 
obtained by means of a dry lens by Gundlach, of tV German inch 
focal length, and the object was illuminated by a triple condenser 
of very wide aperture, made for me by Nachet, of Paris, used with 
a stop which allowed only a small but very bright concentric beam 
of light to fall on the object at a very obhque angle. 
" In the hope that an account of the result which I have thus 
obtained may serve to make more complete the theory of the 
ultimate limit of vision with the microscope, and anxious to obtain 
the opinion of those competent to judge in such a question, I have 
the honour to remain 
Your obedient servant, 
"F. Castkaoane." 
With reference to the facts here described by Count Castracane, 
I wish to offer a few remarks. It appears to me that the visibility 
of the fine lines of Nobert's test-plates depends on several different 
circumstances. The light must be thrown in such a manner 
as to be definitely intercepted by the marking on the glass, or 
they could not possibly be seen ; and we have then to consider 
the efiect of interference fringes, as well as the quality of the 
microscope itself. I do not see that there ought to be any serious 
difficulty in explaining on Helmholtz's principles the resolution of 
Nobert's 19th band. With such an illumination as that adopted 
by Count Castracane, it appears to me very probable that the 
interference fringes would so far coincide with the true lines as 
not to prevent a satisfactory definition. At the same time I am 
anxious to make it fully understood that in my address I en- 
deavoured more to point out the results that would follow from 
Helmholtz's theory, than to examine whether it is or is not in every 
respect true. I should be one of the last to wish it to be looked 
upon as a final solution of the problem. I think many questions 
remain to be cleared up by the actual observations of persons con- 
versant with the theory, and accustomed to the practical use of 
high powers. I am also inclined to believe that several crucial 
tests ought to be examined. Amongst these I would especially 
suggest the study of fine lines at very close yet uneqtial intervals, 
and of lines at equal intervals with one or two missed out here and 
there. Theory indicates that such tests would be far more difficult 
to see correctly than lines ruled at regular and equal intervals ; 
and an examination of such tests ought to afford much information 
respecting both the final powers of our microscopes and the physical 
constitution of light itself. Helmholtz of course assumes the truth 
of the undulatory theory ; and though in the highest degree 
probable, it would perhaps be premature to conclude that it is 
absolutely certain, when applied to the explanation of every 
