Eee eee of the Microscope. 265 
Precisely in the same way, the image of a flame reflected by the 
test globule is a round disk, much larger than it ought to be, as 
seen in all isolated brilliant points in the microscopical field. 
It is interesting then to inquire at what size of image its shape 
can be discerned. And if such small globules be used that the 
shape is gone, and if the real diameter of the spurious disk is a test 
of the correction of the glasses, what becomes of the boasted accu- 
racy of the globule test, in which the size of the spurious disk is 
totally neglected ? No one ever thinks, in testing microscopes, at all 
about this spurious appearance, or takes any thought about its size, 
arising from bull’s-eye iilumination. 
In telescopic testing the most essential part of the ordeal is the 
diameter of the spurious disk. In a case of optical correction 
the microscope cannot be exempted from the same experimentum 
erucis. In the diameter of the spurious disk compared with the 
true image really lies the whole pith of the objective corrections. 
But nothing can exceed the coarseness with which the experi- 
ment is usually conducted. Thus a bull’s-eye condenser, which is 
known to be one of the worst form for spherical aberration, is 
placed before a lamp, and discharges a confused pencil of rays upon 
the test globule, forming a nondescript image upon its surface. 
In cases where errors are reckoned by the 100,000ths of an inch 
a more perfect system is necessary. 
I therefore propose the use of Double Stars imaged upon the 
mercury globule from two brilliant paraffin flames placed edgeways 
towards the quicksilver for a position of maximum brilliance. 
The resolution of the double star must not be considered fair 
(when the disks are merely ovalized or elongated) except they are 
really divorced or sharply divided. The size of the globule, the 
distance of the flames and interval separating them being measured, 
data are at once given for finding the size and separation of the 
minute disks in the double-star test. 
- Globule 
Diameter 
Let G be the globule sj5oth of an inch in diameter and yp}yoth 
radius ; then if the distance of the flames from the globule be fifteen 
inches, and the interval separating their flames ‘be three inches, 
