304 _ Transactions of the [eee ee 
As achromatism theoretically depends not on form or curvature, 
but position and focal lengths of lenses, as well as the refractive 
indices, the new lenses introduce by their movement a new com- 
pensation for different thickness of cover, which, it is well known 
to first-class object-makers, introduces a disarrangement of achro- 
matism, &c. 
There is a certain position, for instance, where the beautiful 
spherules of P. Formosum exhibit, with Powell and Lealand’s jth 
and Searcher, six black dots round each bead. 
The central median beading of Formosum, Angulatum, and 
Rhomboides can only be seen by aplanatic definition and illumi- 
nation somewhat obliquely applied. 
I may now quote from my first paper, read November, 1869: 
—<“T have found an oblique centrical pencil of aplanatic cones of 
light of small aperture (15° to 20°) of the greatest practical utility, 
the obliquity being varied according to the object in view” (p. 297). 
This passage essentially describes an aplanatic condenser free 
from spherical aberration: 7. ¢. a first-class objective stopped off to 
20° or 15° aperture. 
I have been in the habit, for some years, of obtaining aplanatic 
illumination by means of object-glasses of a fine quality; stopping 
off such rays as appeared to produce indecisive and confusing 
shadows. ‘There seems a similarity between the acts of drawing 
and defining. The masterpiece of the artist, though embodying 
form, is essentially dependent on a judicious selection and distribu- 
tion of light and shade. And he only among microscopists who can 
best manage illumination by the shadow regulation, will best succeed 
in deyeloping the most difficult structures. The more perfect, sharp, 
pure, and decisive are the shadows, the greater assistance will be 
given to a pure definition, banishing as much as possible the useless, 
the worse than useless, rays producing counter-shadows. 
In the early part of the present year I had the pleasure of 
exhibiting to our President, the Rev. J. B. Reade, F.R.S., Mr. 
Brooke, F.R.S., Dr. Millar, Mr. Slack, F.G:S., &c., &c., my method 
of oblique aplanatic illumination, with either a 2-inch, 14 or 1-inch 
objective. In order to ascertain that the illuminating pencil is 
passing centrally through the stop, I place red paper upon the 
front glass, perforated with an aperture either small and cireular or 
V-shaped, quadrantal, or even semicircular, varying the inclination 
and position of the pencil by stop-motions designed about six years 
ago. I had the pleasure of exhibiting the whole mechanical action 
to Mr. Browning so early as the spring of 1869. 
More recently a well-known worker upon microscopic improve- 
ment has brought out a similar method of illuminating by means of 
object-glasses (Dr. Mathews). The smaller and more perfect the 
pencil, the sharper are the shadows of the things to be defined. A 
