Monthly Microseopia'] Royal Microscopical Society. 299 
The spurious disk is that identically the same with the smallest 
circle of greatest condensation of converging rays. 
Here the subjects of illumination and definition obtain a common 
base of operation. Indeed, the same diagrams and facts which 
illustrate the one apply equally to the other. (Diagram I., Plate 
LXVII.) 
If conical pencils proceed from Q, and are reflected by a concave 
mirror obliquely, a series of foci are formed. Confused images are 
produced of great complexity: and that space or section through 
which all the reflected pencils pass is a locus of the greatest con- 
fusion and decussation. 
Hach cone of light produces two foci-—long and short—primary 
and secondary. ‘The art of microscopy often consists entirely in 
selecting the most favourable points of the confused decussation to 
produce the desired effect. I say desired, because the microscopist 
more than any other observer is under temptation to produce 
special effects, dealing as he often does with the really unknown. 
An oblique illumination with a concave mirror is the worst 
possible source of illumination imaginable from a brilliant point or 
definite origin of light. Hence the value of the white cloud. 
Aplanatic illumination is impossible from an oblique spherical 
mirror. Indeed, even with the parabolic form of Browning's silver 
glass mirror the slightest obliquity gives a strong wing or flare to 
an observed star or coma—a sufficient proof of the well-known 
destruction of aplanatism. The image is no longer clear of aberra- 
tion or wandering rays. But if we turn to the property of the ellipse 
the lines Q P, F P, drawn from the foci, are equally inclined to the 
tangent at P, P R.* Every ray shot from Q against the internal 
curvature will accurately cross the axis at one and the same point, 
F’, and the two foci are absolutely aplanatic to one another. 
In Diagram III. an image is formed by an infinite number of 
spurious disks corresponding to each point of the object. If a plate 
stencilled with small apertures in outline be employed, the spurious 
disk of each will appear expanded, and coalesce into one continuous 
image, which is very imperfectly defined by a plano-convex lens, 
when the convex side is turned towards the image. Reversing the 
lens, the image is improved ; if a double convex lens be used, and 
the image be formed of the same size as the object, by arranging each 
equidistant from its centre the spurious disks will more nearly equal 
the true. 
I now turn to a novel experiment (at which I feel sure every 
one that performs it for the first time will be greatly surprised), by 
which spurious disks of rare and brilliant beauty and significance are 
developed with the most gorgeous hues. ‘The experiment requires 
* Q and F are the foci, and P a point in the curvature, 
