Capacity of the Microscojpe. By Prof. HelmhoUz. 19 
that he has written. And he also cordially recognizes the high 
aim and zealous study of Dr. E. Pigott, the direction of whose 
labours must ultimately prove most serviceable to all who desire 
to understand the real power and possible perfection of their 
favourite instrument. Any unfair spirit of criticism of matters so 
little appreciated by some of his critics is to be earnestly depre- 
cated. One can only regret, whilst profiting by the opportunity 
of hearing all sides of a question, to be reminded of the woeful 
sentiment "tantsene celestibus irse." The vexatious partisanship 
of aperture " and the disputed estimates of the performance of 
lenses constructed by this or that maker, must appear as over- 
strained and even ridiculous to the optician who can best gauge 
his own or any other maker's work, as to those who care only to 
understand the principles of construction and to form a rational 
judgment of their action. 
It is to be hoped that a more general agreement on the essen- 
tial parts of the theory of the microscope will soon prevail, and that 
the exaggerated significance of certain matters too long discussed in 
our journals, will fade to its proper vanishing point. 
The Theoretical Limits of Optical Capacity of the Microscope. 
In Poggendorff's ' Annalen ' for 1874, Professor Helmholtz 
pubhshed an article, of which the following is a translation. 
Whether, and to what extent, the optical performance of the 
microscope is capable of further improvement, is a question of the 
greatest moment for many branches of natural history. Doubtless, 
some progress, and notably through the revival of Amici's sugges- 
tion of immersion lenses adopted and carried out with such success 
by Hartnack, has been made, but each onward step is slow and 
faltering. We have, it is clear, arrived now at a point at which 
any trifling gain is effected with a disproportionate effort of mental 
as well as mechanical labour. And yet, so far as I caA see, no one 
has been able to give any reason why this should be, excepting the 
common belief that the difiiculty lies in overcoming the spherical 
aberration of lenses so small and of such quick curvation as is 
needed for objectives of very high magnifying power. It is not 
long since Herr Listing, one of the most eminent authorities on 
this subject, discussed * the means by which it might be possible 
to obtain amplifications ranging from 25 to 50,000 diameters, 
whilst in actual practice the ordinary range of serviceable ampli- 
fication is at the present moment limited to from 400 to 800 
diameters. Moreover, the collective experience obtained by re- 
peated efforts of practical opticians has taught us that all high 
amplifications combined with good definition (i.e. sharp delinea- 
* Poggendorff's 'Ann.' vol. cxxxvi. 
