Royal Microseopieal Society. 
59 
There is another point of view worth considering, viz. the 
appearance of briUiantly reflecting particles under illumination 
from above. The peculiar invisibility of minute beading under ordi- 
nary wide-angled glasses under reflected light is quite as striking 
a result as that given by transmitted light. A brilliant scene is 
lit up upon dark ground. The appearances presented remind me 
of the beautiful eflects displayed in the field of view of a first-rate 
telescope directed to a dew-drop ghttering in the morning sun- 
shine. 
No glass yet constructed, whether microscopic or telescopic, has 
yet been adequate to present to the eye the real size of the image 
of the sun seen on a small spherule.* 
The study of Mr. Slack's films by reflected light on a black 
ground indeed well repays the observer. Eich fields of sparkling 
beauty, variegated with tiny stars of various magnitudes down to 
exquisite groups of star-dust as it were closely resembling resolved 
nebulae and cloudlets of nebulosity, doubtless indicating beading 
still more minute and undefinable— such are some of the lovely 
pictures formed by these films. 
Precisely in the same way, thus illuminated, the spherical silica 
beads present very beautiful diffraction rings according to the 
quality of the glass, and the Zeiss " sixth " certainly gave very 
finely formed concentric ones. On trying a badly - corrected 
eighth objective, and thought " fine " at the time by the makers, 
the brilliant speck reflected by a single bead presented an exact 
representation of Saturn and his ring as it were viewed perpendi- 
cularly to the plane of the ring, no division being visible : in fact 
there were no delicate diffraction rings whatever. 
Now in wide aperture glasses it is possible many images may 
be embraced by the extreme rays of the objective : just as each 
person views at one and the same time a different set of solar 
rays reflected from the falling rain-drop. 
It would seem that an extremely wide-angled objective is not 
adapted for defining brilliant points of light reflected from minute 
spherical surfaces. Nor is it so well adapted for developing the 
aperture test-band of solid highly-refracting particles. On the con- 
trary, it completely hides it. 
I shall beg leave to conclude this note by a further quotation 
from the article already cited,! (in the case of transmitted light) — 
" From these effects of aperture it may now be assumed that 
* With the telescope a disk which ought to be the two-thousandth of an inch . 
appears something like the fortieth of an inch in diameter : or the spurious disk 
is 500 times larger than the reality. It is my opinion, however, from many care- 
ful experiments that microscopic object-glasses are more finely constructed than 
the telescopic, and that great improvements are still necessary in that direction. 
t 'Quart. Journ. Micro. Sci.,' Jan., 1870, "Kesearches on the Errors of Micro- 
scopical Vision and on New Methods of Correcting them," by Dr. Koyston-Pigott. 
VOL. XIII. F 
