236 Transactions of the Royal Microscopical Society. 
experiments is to prove that small angles can do much more in the 
way of resolution than has been commonly supposed, and that few 
objects require for their finest exhibition as large angles as are 
usually given to the most carefully made high powers. When a 
high degree of resolving power is obtained by large angles, the 
objectives necessarily fail in every instance in which the surfaces it 
is desired to examine are not very flat, or cannot be placed exactly 
in a horizontal plane. Were more skill exerted in the construction 
of smaller-angled glasses, the instances would be_ very few in 
which they would fail to show markings for which large angles are 
thought indispensable. 
Professor Abbe, of Jena, in a paper which will be found in 
Schultze’s £ Archiv. f. Mikroskop. Anat.,’ vol. ix. (1873), is entitled 
“ Beitrage zur Theorie des Mikroskops,” observes that we should 
look for improvement in the direction of making objectives of 3 and 
4 millimetres focus do the work now done by higher powers. He 
affirms that corrections cannot be well made with dry lenses ex- 
ceeding 105° to 111° aperture, without a considerable reduction of 
working distance. This is much in accordance with the opinion of 
the late Bichard Beck, that it was not well to give an |th a greater 
angle than 120°, and the Aobh constructed by him was of about 
that aperture. Immersion lenses, Dr. Abbe says, allow of good 
correction to an air valve of 180°, giving, according to Zeiss’ 
catalogue, from 104° to 108° in water ; and an objective that corre- 
sponds with our English ^th has the power of working through 
a covering glass ^th mm. thick.* 
With regard to Zeiss’ immersion systems, -|th, T Vth, and ^th, 
of about 100° aperture in water, he says : “ Personally, I am con- 
vinced that even in immersion systems, for the normal requirements 
of science, there would be no loss, but in many respects a gain, if 
they were constructed with smaller angles of aperture, although 
we cannot suppose that practical opticians will exert themselves in 
this direction while, according to a universally spread opinion, such 
objectives would be valued as of only second rank.” 
It is this absurd mode of valuing objectives that is now the 
greatest hindrance to further progress. An optician can get great 
credit for giving a | inch an angle too much for a ^sth, but might 
get no credit for constructing a quarter of moderate angle so perfect 
in correction as to possess the resolving power associated with a 
large-angled and yet to accomplish the latter would be a feat 
of higher skill and of much greater usefulness. 
No English optician is known to the writer as now attempting 
this task, and we must refer to Zeiss’ productions for illustrations 
of what has already been accomplished. There are, however, small- 
* The millimetre is equal to 0 - 039 of an inch, one-fifth of which is nearly 
tItt inch. 
