148 Transactions of the Royal Microscopical Society. 
perfection, how much genius, patience, and manipulative skill, 
knowledge of the various kinds of glass and curvatures required, 
the makers of such perfection can alone declare. The continual 
abandonment of old forms and combinations, formerly thought the 
ne plus ultra of art, and the adoption of new, sufficiently vindicate 
a former declaration that perfect corrections had not then been 
attained ; hut that the future is full of hope is shown by recent 
improvements still made in the face of such refined difficulties. 
In my first paper, Dec. 1869, I stated the residuary aberration 
in the best glasses might be reckoned at the 50,000th of an inch. 
This small matter seems insignificant ; yet in defining a single head 
the sixty or even the thirty thousandth of an inch, such an error 
cannot he despised. 
In the older glasses, the effect of this aberration (often much 
larger than this) was to cause a series of heads to run into each other. 
I have ever found, as the definition became more and more refined, 
these objects appeared smaller and smaller with the same magnify- 
ing powers. Our Honorary Secretary, Mr. Slack, noticed recently 
that in Powell and Lealand’s new construction (not yet issued) the 
heading of the Angulatum appeared under a power of 4000 with 
an eighth much smaller than usual: in short, more “ widely 
divided 
A had glass, it may now he stated, unduly enlarges a point : if 
it be black, it is diluted as it were into a larger and paler area than 
the truth. In the same way a fine line, which is an assemblage of 
points, appears thickened and blurred : I have even here noticed 
dark diffraction rings * surrounding dark points. 
If the point he brilliant, it has special developments as to its 
appearances as the glasses depart more and more from perfection. 
And these developments take their own peculiar forms according as 
the point is itself an origin of light — a reflector or a transmitter of 
light. They also vary with the directness or indirectness of illu- 
mination, and with the position of the line of vision. 
If a globule of mercury of exceeding minuteness is placed upon 
the stage, illuminated as best we may, and then be viewed with a 
T ^th, only a very imperfect picture of the illuminating flame is 
exhibited by the globule, in consequence of the great inconvenience 
of the closeness of the objective to the stage and the limited range 
* These may often be seen in gold leaf placed between two glasses and 
illuminated by sunlight by transmitted light. The beautiful malachite green of 
the film of gold will be seen perforated in many places with apertures of any 
degree of minuteness, each displaying diffraction rings ; the best objectives will 
show an intensely black border round each aperture, and as the focussing is 
changed, over the general surface dark points will be seen to swell and shrink, 
and dark diffraction rings may be noticed correspondingly to expand and contract 
with great regularity. The behaviour of a good glass may be strikingly con- 
trasted with that of an inferior quality when tried upon this gold-leaf test, which 
I strongly recommend, especially when transmitting solar light. 
