ae ora eee A ih of the Microscope. 263 
surrounded with six black round dots; the power was about 4000 
diameters, and a similar objective was employed. 
In my hands the immersion y,th has displayed the beading of 
which the median ribs of the 
Formosum (a) .. (Diameter, three to one) 
Angulatum (6) .. (Diameter, three to two) 
and Rhomboides (c) .. 5 7 
appear to be composed. Very curiously the diameter of these beads 
bore a different proportion to the general beading in each case, as 
above indicated ; a confirmatory fact for the truth of their existence, 
and negativing the supposition of their being spurious, and guaran- 
teeing their integrity. 
Ture New Dovsie-Star AND ImAcE TEsts. 
Before describing these tests, it will be convenient to lay down 
an optical principle upon which they entirely depend. 
The smallest section of any converging pencil of rays is called its 
circle of least aberration. 
If a brilliant point of the utmost minuteness be examined with 
a high-power microscope, it will always present a spurious disk 
more or less surrounded with coloured rings and diffraction rings. 
This disk, the best image of a minute point, is always larger than a 
perfect image. It is this effect which enlarges and confuses minute 
refracting molecules, which gives a blur to a sharp terminal 
boundary of a scale, and which renders individual spherules so 
difficult of perfect definition. It is only with the very finest glasses 
that these common effects are diminished. A BURR envelops the 
point of light. And lastly, the spurious disk is even in good glasses 
more than double the true size of a perfect aplanatic image.* 
The next point to be stated is that an image is an assemblage 
of circles of least confusion corresponding to every point in the 
object. 
; It follows from these principles that the slightest deviation from 
aplanatism or perfect focalization produces an enlarged spurious 
disk instead of a perfect image of any, the minutest point of light, 
considered as an object: the object and image being of course placed 
at what are termed the conjugate foci. 
Tae New Dovste-Star Txsty 
Taking Mr. Wenham’s further objection as a text:—“In this 
inquiry it is remarkable how the use of the mercury globule is 
* T once heard an optician declare that aplanatic meant a flat field! The 
perfect meeting of all the converging rays in an absolute mathematical point is 
of course the proper meaning. 
