258 Royal Society :— 
power,— instead of depending upon the deepest eyepieces and most 
powerful objectives hitherto “constructed, that better effects cou 
be produced by regulating a more gradual bending or refraction of 
the exeentrieal rays emanating from a brilliant microscopic origin 
of light. 
It then appeared that delusive images, which the writer has ven- 
tured to name eiddla*, exist i close roximity to the ben focal 
point (where the least circle of confusion finds its locus), and :— 
I.) That these images, possessing extraordinary cha iite exist 
principally above or be elow the best focal point, accordingly as the 
objective spherical aberration is positive or negative. 
That test-images may be formed of a high order of deli- 
cacy and accurate portraiture in miniature, by employing an objective 
of twice the tum depth, or, rather, half the focal length of the ob- 
serving-object 
CIII.) Phat ih test-images (which may be obtained conveniently 
two thousand times less than a known original) are formed (under 
precautions) with a remarkable freedom from Boa ae which 
appears to be reduced in the miniature to a min 
V.) The beauty or indistinetness with Eh. they are displayed 
fores on the immersion system) is a marvellous test of the cor- 
of the observing objective, but au indifferent one of the 
Refs PST objective “used to produce the testing-miniature. 
These results enable the observer to compare the known with the 
re ha By observing a variety of brilliant images of known 
objects, as gauze, lace, an ivory thermometer, and sparkles of mer- 
secrecy properly adjusted so that the axes of the two objectives 
m incident and their corrections suitably manipulated, it is 
SELE to eompare known delusions with suspeeted phenomena. 
It was then observed (by means of such appliances) that the aber- 
ration developed by uum -power eyepieces and a lengthened tube 
followed a peculiar la 
A engthened mibe increased aberration fastet than it gained 
power. (Roughly, the aberration varied as v?, while the power varied 
as v 
he image was formed by the objective at points nearer to 
it than the standard distance of nine inches, for which the best 
English glasses are corrected, the writer foun the aberration dimi- 
nished Erg than the power was lost by shortening the body of the 
instrum 
: e aberration became negatively affected, and required a 
* eerta cidwXor, a false spectral image. 
