171 7 
size of images of known objects at known distances, the 
examination of immersion objectives was next undertaken. 
Double stars were artificially produced in thin brass (3, of 
an inch thick) by placing mimute apertures (,32, in dia- 
meter) in front of a brilliant flame, at the distance of 100 
inches from the focal point of observation (fig. 9, Plate IX). 
The apertures were so arranged as to gradually exhibit closer 
double disks (as shown roughly in fig. 9), which were care- 
fully drawn on brass under the microscope and then accu- 
rately pierced. ‘The miniature effect of the star-doublets is 
represented in the following Table, the immersion one six- 
teenth objective being converted into one twentieth,! so that 
f here=2,, m=2000 at 100 inches distance (nearly). 
Calculated size of Distance between 
Doublet. Size of disks. images nearly. their centres. 
12 1 
No. 2. 1000 166000 S000 0 
org: cies ibe 3 hy 
= 1000 1 6000 20000 
It will be readily seen from the diagram (fig. 8) that, in 
No. 2 the disks being ;42, and the separating interval be- 
tween centres being =, the actual dividing interval is -34,, 
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,” it was found that the 
separating interval (accurately measuring ;3%, of 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 Bin fig.6 & BC fig.8. Apparently, 
therefore, in the eyepiece, spurious disks had been formed 
four times and one sixth larger than a true aplanatic repre- 
sentation by the microscope.’ 
It follows from this experiment that if the disks be sup- 
1 By the adaptation of a “ water-lens” one-thirtieth inch focus. 
2 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 inseparable from this experiment, as already explained. 
The minute apertures, made accurately with Swiss watchmaking tools, 
were carefully Baabened: to prevent internal reflexion, with a solution of 
perchloride of platinum. 
VOL. XI.—NEW SER. M 
