On a Large-angled Immersion Objective. By J. W. Stephenson. 53 
glass has been fixed. By plunging the prism into the fluid, the 
refractive and dispersive properties of any oil or mixture of oils can 
be determined and adjusted by simply viewing the bar of a window 
through the liquid and the prism. 
The qualities of the new objective may he thus briefly de- 
scribed : — 
1. There being no aberration to correct for varying thickness 
of cover-glasses, there is no collar adjustment. For thick covers, 
say 0*008 to 0*009. the ordinary length of 10 inches gives the 
most perfect definition ; for thin covers, say 0*004, a length of 
12 inches is perhaps better. But the difference is so very slight 
that it is scarcely necessary to use the draw-tube. 
2. It has a balsam angle of 113^ = 1 *25 numerical aperture, 
which renders it extremely sensitive in focussing, and also indi- 
cates the highest resolving power hitherto attained. 
3. It has a large working distance. The distance between front 
of lens and object is 0 * 02, which gives a working distance of 
0*012 for 0*008 cover-glass, 0*016 for 0*004 and so on. 
4. Its power is rather more than one-ninth, and having com- 
ponent lenses throughout the combination, larger than in other 
objectives of the same power, it transmits more light, the latter 
quality being enhanced by a diminution in the reflection of the 
peripheral rays. 
5. It bears very deep eye-pieces and has a flat field. 
Lastly. An essential condition to its perfect performance is that 
if the object be dry, it must be mounted on, or nearly touch, the 
cover, or if not a dry object, that it be mounted in some medium 
having approximately the same refractive index as the oil, such as 
Canada balsam, &c. 
The special advantage of this objective for petrographic work 
is, that the oil used, having very nearly the refractive index of the 
objects to be examined, renders cover-glasses and highly polished 
surfaces unnecessary ; the minerals, if sufficiently translucent, can 
be observed through a considerable depth, say 3^0 of an inch, and, 
on the assumption of identity of index, every plane, from the sur- 
face downwards, will have the same perfect correction. 
To a certain extent the latter observation applies to all trans- 
parent objects mounted in balsam, as the thickness of the balsam 
above the object may be looked upon as equivalent to a thicker 
cover, and the penetration must therefore be considerable, although 
the latter quality will be still greater in the smaller-angled 
objectives constructed on the same principle, which will hereafter 
be made. 
Professor Abbe, writing to me on the objective, says : — “ The 
advantage of the greater aperture is shown in a most striking 
manner on Pleurosigma angulatum, when observed by very oblique 
