172 
posed to gradually diminish to points, the limiting value of 
the residuary spurious disks would give nearly =>},% of an 
inch for the diameter of the least circle of confusion, repre- 
senting the actual amount of residuary lateral aberration. 
This appears from a diagram, where in the limit (fig. 8) 
EF = 4AE, when AB,CD both vanish in the case of the disks 
being reduced to points. : 
The phenomena presented by these artificial doublet-image 
tests, gave fine evidence of the skill employed in the con- 
struction of the glasses, and of the accuracy with which the 
axes of the optical parts had been made to coincide in this 
delicate experiment. 
All the disks appeared sharply cut and planetary (fig. 10), 
surrounded with a black ring supplemented by accurately 
formed diffraction rings, which enlarged and glowed with 
prismatic colours, both within and without the sharpest focal 
point or image, forming concentric intersections, displaying 
coloured pencils passing either to or from their finest point 
of focal combination where colour should be destroyed 
fig. 6). 
Experiment 3.—The disks (A) shown by the apertures 
;+4, of an inch in diameter, and separated between centres 
a5, were now brought nearer to the objective O'. It was 
then observed that the image-disks (of above four times their 
proper size) began to separate; and since the spurious disk 
retains its false annular expansion independent of the true 
magnitude, it became evident that the exact distance at 
which the test doublet was first divided, gave for other 
objectives a comparative measure of their aberration; the 
very slight aberration in the image (of the sixteenth) being 
scarcely appreciable, especially when favoured by the advan- 
tage of the water-film to enhance the precision of definition 
on the immersion system. 
By such experiments, with the finest glasses obtainable, 
the existence of an aberration of material and measurable 
amount being thus established, the next question to be settled 
assumed the following character, viz. 
What was the nature of the aberration produced by dis- 
placement of the final focal image viewed by the eye-lens ; 
and whether better effects could be produced by a different 
distribution of the magnifying powers. 
It was now found that increasing the distance between 
the eye-lenses and the objective, gained power indeed, but 
caused the aberration to increase faster than the power 
gained. 
Intermediate Huyghenian eyepieces, inverted, were found 
~ —_ 
