52 
Transactions of the Royal Microscopical Society. 
characteristic energy at once applied his great knowledge to the 
solution of the problem, and with complete success. 
He informed me that he had already from another point of 
view considered the propriety of using a more highly refractive 
medium than water for “ petrographic ” work (Diinnschliffe), as 
the thin plates of minerals ordinarily used for microscopic inspec- 
tion are generally so roughly cut and insufficiently polished that 
their observation with high powers is very difficult, if not im- 
possible. 
The more general problem of constructing an objective in which 
the necessity for correction, by an alteration of the distance between 
the lenses, could be entirely dispensed with, was the more attractive 
to the Professor as the use of a more refractive medium than water 
enabled him to deal with higher apertures, without loss of definition, 
than had hitherto been attained. The result of his calculations was 
the construction by Mr. Zeiss of an objective of excellent quality, 
satisfying the conditions named and having a balsam angle of 
113°= 1 • 25 of “ numerical aperture ” ; an aperture which is, if I 
mistake not, larger than any that has been hitherto produced, and 
one which coincided exactly with the theoretical value given by 
Professor Abbe’s formula. But here an unexpected difficulty arose : 
it had been assumed by Professor Abbe and myself that amongst 
the numerous fluids suitable for use, there w^ould readily be found 
some, pure or mixed, which would give exactly the same refraction 
and dispersion as crown glass ; this, however, proved not to be the 
case, and the difficulty was not overcome until he had tested no 
less than sixty- three different oils, and nearly thirty other fluids : 
he at length ascertained that oil of cedar wood,* although not 
absolutely identical with crown glass, was admirably adapted for 
the purpose, giving perfect definition with oblique light, but, for 
central light, being improved by the addition of one- fourth or one- 
fifth of oil of fennel seed [01. Foeniculi). With central light the 
difference between the spherical aberration of the green and red 
rays gave, when pure cedarwood oil was used, too much colour, 
wffiich was corrected by the addition of a small quantity of the 
more highly dispersive fluid. In the course of these nuoierous 
experiments it was found that the tables given by Brewster and 
Wollaston were very unreliable, nearly all the refractive indices 
given by those authors being considerably too high, and most of 
the dispersion values too low. As it is probaWe that further 
research may still result in finding a more perfect medium than 
that now used, I may mention that Professor Abbe determines the 
suitability of a medium by using a bottle, the sides of which are of 
parallel plate glass, and to whose stopper a small prism of crown 
* The asphaltc varnish on the slides can be protected from the dissolving 
action of the oil by a coating of sealing-wax varnish or gold size. 
