veea ., ‘ Monthly Mi ical 
300 . Transactions of the penny ys 
oblique solar rays froma plane mirror. Having arranged a diatom 
(the Pleurosigma Formosum was the one selected) with its awis 
towards the oblique source of light, and having developed the lines 
with a good objective, so as to show the structure, cut off the ex- 
centrical rays by means of the Aberrameter. 
Immediately a gorgeous coloured disk can be seen—perhaps four 
times as large as the individual beading. Now traverse the diatom 
slowly across the field, lengthways ; immediately the brilliant disk 
appears to roll in a contrary direction, each bead appears in suc- 
cession (telling a wondrous tale of internal action). With intense 
vividness of colouring, the rolling disk resembles a setting sun, 
travelling on the horizon with the motion of the observer in a 
rapid train; whilst its splendour glows rapidly through all the 
prismatic colours. Only one disk is visible at once; a new one, 
with a new colour, starts forth in place of the old at the slightest 
movement of the object across the microscopic field. 
Without commenting upon this magnificent and lovely pheno- 
menon further, than to say that it is evident each bead acts as a 
refracting sphere, or lens, forming in its focus an image of the sun 
spuriously enlarged, I pass forward to Diagram III. 
- Parallel solar rays converge to a focus at F. Opposite the 
arrows { the rays, in their decussation, pass through the smallest 
ring. ach ray has here been carefully laid down from the Devia- 
tion Tables, refractive index being - : 
In Diagram III. the caustic curve formed by the intersection of 
the refracted rays, and to which curve they are successively tan- 
gents, is shown on an enlarged scale. A spherule zvoooth of an 
inch in diameter, being represented by a segmentary portion, the 
sphere being delineated 5 feet in diameter.* The short arrows 
denote the position of the least circle of aberration. 
The central rays cut the axis at F, the ultimate, or solar focus 
—distant 1th of the diameter from the surface of the sphere. Some 
of the rays are lost by internal reflexion. 
_ It will be seen by examining the diagram that as the aperture 
is cut off, the smallest ring is found constantly nearer the final 
focus F. The smallest rmg through which pencils from a given 
aperture can pass is the least circle of aberration ; and here it only 
is reduced to a point where the aperture is exceedingly diminished. 
The diameter of the spurious disk of aberration is here about a 
small fraction of the diameter of the refracting spherule. But in the 
prismatic effects just described, the spurious disk of an image of the 
sun was at least four times the size of the bead or lens producing it: 
producing the very remarkable phenomena already alluded to. 
* In the diagram exhibited at the meeting. 
