( 51 ) 
IV. — On a Large-angled Immersion Objective, without Ad- 
justment Collar ; with some Observations on “ Numerical 
ApertureC By John Ware Stephenson, F.E.A.S., and 
Treasurer of the Eoyal Microscopical Society. 
{Bead before the Eoyal Microscopical Society, April 3, 1878.) 
In the use of the microscope few things are perhaps more difficult 
than the effective application of the adjustment collar of a large- 
angled modern objective. 
Not only is the best point of correction difficult to attain, but 
in too many cases, either because the covering glass is too thick or 
too thin, the slide is not suitable, and it is impossible to do justice 
to the probably otherwise excellent quality of the lens. 
In fact, so indeterminate is the best point, that an objective 
thrown out of adjustment would seldom be brought back to the 
identical number on the scale, if readjusted, by any ordinary 
observer, even on a well-known object. 
If this be so, on an object with which one is familiar, how much 
more difficult must be the examination of an object the structure 
of which is perhaps wholly unknown. 
Such considerations suggested to the writer the great de- 
sirability of constructing an object-glass in which cover correction 
could be entirely dispensed with. 
The origin of the correction collar as shown by the late Andrew 
Eoss, was the imperative necessity of compensating the error 
arising from the difference between the refractive index of the 
covering glass and that of the air between the front of the lens and 
the thin cover, whenever a high power was used. 
Now it is evident that if some fluid, of which the refractive and 
dispersive powers are the same as those of the covering glass, were 
substituted for air in the intervening space, the end in view would 
be attained. 
If, then, such a medium could be found, with, as I said before, 
optical properties identical with those of the thin cover, it follows that 
as the distance between the front lens of the objective and the under 
side of the cover (or the object adhering to it) must, of necessity, be 
always the same ; that is to say, as a thick covering glass would 
require a thin fllm of the proposed immersion fluid, and conversely 
as a thin cover would require a thicker layer (one, in fact, being 
the complement of the other) ; the two together would have a 
constant value, which would be exactly equal to the focal length of 
the objective. 
From this point of view the writer suggested to Professor Abbe 
the possibility and desirability of the construction of a combina- 
tion satisfying the conditions named, and that gentleman with 
