58 
Transactions of the 
refractive power and variable aperture upon the character of the 
defining band developed. 
Thus the peculiarly pale appearance at the edges of some 
silicious forms found in sponges under ordinary apertures is de- 
pendent upon their very low refractive index. Indeed, Tabsasheer 
will not develope the defining band at a greater objective aperture 
than 25°. 
I have not as yet measured the refractive index of Mr. Slack's 
silica beads, but suspect it is very low. For this reason alone, a 
very high-angled objective would fail entirely to detect the circular 
defining band of a very minute spherule. That gentleman has 
kindly presented me with several of his slides, and left in my care 
an excellent Zeiss ^ from Jena. Considering the fine performance 
of this glass notwithstanding it has no adjusting collar, I have in- 
stituted experiments on its aperture : and at the same time measured 
that of Powell and Lealand's glasses. 
Objective. 
Focal 
Length. 
Angular 
Aperture. 
Character. 
Powell and Lealand . . 
Ditto ditto 
1 
8 
* { 
68° 
98° 
124° 
124° 
Dry lens. 
Dry, latest construction. 
Immersion, ditto. 
Immersion without water. 
The angular aperture was measured by laying a tube, into which 
was screwed the objective, upon a large flat board, then two night- 
lights were placed at about a foot distance from the nose of the 
object-glass : the lights were then gradually separated till, upon 
looking through the tube, both appeared distinctly in miniature at 
the extreme edge of the field. On replacing the eye-piece and 
alternately hiding each, the field of view was symmetrically illumi- 
nated. Two lines were then carefully ruled from the centre of the 
front to each fight, and the angle subsequently measured by a pro- 
tractor. 
In the case of Powell and Lealand's " 1872 eighth," the angle 
was first measured dry, and then a small piece of covering glass 
being wetted was attached to the front lens : in each case, as might 
be expected, the same oblique rays reached the observer's eye at the 
same position of the lights, viz. 124°. 
I may remark here that one of Andrew Boss' finest " quarters " 
had about the same angle, tested in this way, as Zeiss' " sixth." 
This mode of testing is altogether different from measuring the 
angle at which a ray of light emanates from a brilliant particle 
itself immersed in a highly refracting medium : the two things are 
totally distinct. 
