DE. G. W. EOYSTON-PIGOTT ON A SEAECHEK EOE APLANATIC IMAGES. 595 
distance of 100 inches from the focal point of observation (fig. 9, Plate LII.). The 
apertures were so arranged as to gradually exhibit closer double disks (as shown 
roughly in fig. 9), which were carefully drawn on brass under the microscope and 
then accurately pierced. The miniature effect of the star- doublets is represented in the 
following Table, the immersion one-sixteenth objective being converted into one-twen- 
tieth*, so that/’ here = 2 ^, m= 2000 at 100 inches distance (nearly). 
Doublet. 
No. 2. 
No. 3. 
Size of disks. 
1 2 
lOOO 
1 2 
lOOO 
Calculated size of images 
nearly. 
1 
166000 
1 
1 6 6 0 0 0 ' 
Distance between 
their centres. 
40000 
l 
20000 
It will be readily seen from the diagram (fig. 8) that, in No. 2 the disks being and 
the separating interval between centres being -^y, the actual dividing interval is toIrp or 
above three times the real diameter of each disk. 
Experiment 2. — A drop of distilled water being suspended between the objectives, 
both of which were fitted with single front “immersion lenses,” I was astonished to find 
that the separating interval (accurately measuring ytyfo °f an inch) between the centres 
of the disks had totally disappeared in the miniature image. The disks now resembled 
a finely divided double star just separated by a black line, yet this minute interval should 
have appeared above three times the diameter of a disk, as at B in fig. G & BC fig. 8. 
Apparently, therefore, in the eyepiece, spurious disks had been formed four times and 
one-sixth larger than a true aplanatic representation by the microscope f. 
• It follows from this experiment that if the disks be supposed to gradually diminish to 
points, the limiting value of the residuary spurious disks would give nearly ^ hoo °f an 
inch for the diameter of the least circle of confusion, representing 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 construction 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 
* By the adaptation of a “ water-lens ” one-thirtieth inch focus. 
t It will be observed in this experiment that the standard distance of 9 inches at which the object should 
be placed from the objective was increased to 100. It was found that only a very slight adjustment of the 
screw collar of the image-objective was necessary to compensate for this great increase of defining distance. It 
is hardly necessary to remark further that in a minute miniature image the aberration is insignificant compared 
with that taking place in a greatly magnified image of an object placed in the focus. This distinction is inse- 
parable from this experiment, as already explained. 
The minute apertures, made accurately with Swiss watchmaking-tools, were carefully blackened, to prevent 
internal reflexion, with a solution of perchloride of platinum. 
