ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
211 
is quite incapable of fully developing the image given by an oil-immer- 
sion apocbromatic with a large optical index. In opposition to this it 
may be urged that most biologists and histologists use Microscopes of 
the Continental pattern ; to which I reply that biologists and histologists 
always work their objectives down, and never up ; by this is meant that 
an oil-immersion 1/12 is used where a 1/2 or a 1/4 ought to be sufficient. 
A lens used with a small cone is a lens * degraded.’ 
“ Now, as a general rule, biologists and histologists use the tliree- 
lens Abbe chromatic condenser, which only yields an aplanatic cone of 
0*5 N.A., consequently an oil-immersion 1/12, however great its excel- 
lence, can only be used as a ‘ degraded * lens with such a condenser. 
A fine adjustment however is only put on its metal when a 3/4 cone 
is employed ; you may be perfectly certain therefore that those Micro- 
scopists who are content with the efficiency of the direct acting screw fine 
adjustment, never employ a 3/4 cone, and use only £ degraded ’ objectives. 
“ Far be it from me to minimise the value of the work done by any 
biologist or histologist, but I think that the above remarks are called for, 
because it is tacitly assumed that, if any microscopist has discovered any 
biological object with a lens 4 degraded,’ therefore that method and 
apparatus must necessarily be capable of performing the highest and 
most critical microscopical work. 
“ Another misconception also is rife, viz., that the highest type of 
instrument must be the combination of a dissecting and observing 
Microscope ; for we have been repeatedly told that because Strauss 
Durkheim designed, for the dissection of the Coleoptera, a Microscope 
which had the level of its stage four inches above the table, therefore 
four inches is the proper height for all Microscope stages. This is 
wholly an error, because the combination of two incompatible things 
cannot yield the best possible results for each. The best observing 
Microscope will never make the best dissecting one, and vice versa . 
“ In this Microscope we have an interesting feature, viz. an early 
example of the ‘ Jackson ’ limb. The funny thing is that what is now 
known as the ‘Jackson’ limb was not invented by Mr. Jackson ; for it 
was Mr. J. J. Lister who designed this form of limb, and it first ap- 
peared in a Microscope made for Mr. Lister by Tulley. The Microscope 
was completed on May 30th, 1826. Mr. Tulley states, in a pamphlet 
published about that time, that ‘ . . . the instrument and its apparatus 
was suggested and made from original drawings by my friend J. J. 
Lister, Esq., whose ingenuity and skill in these matters are very generally 
acknowledged.’ It should be also noted that Mr. Lister’s Microscope 
had ‘ a combination of lenses to act as condenser under the object ’ ; 
it also possessed an internal lens to erect the image ; in this last point 
clearly predating Strauss Durkheim. You will also notice that the 
tube-rack coarse adjustment, and the limb of Powell’s iron Microscope 
are copied from Mr. Lister’s instrument which, like Powell’s, also had 
no fine adjustment. 
“The question now arises, w r hat wafc Mr. Jackson’s invention? It 
consisted in the ploughing of the slide which carried the body and that 
which carried the substage, in one plane, and out of one solid piece of 
metal. In this connection it is stated,* that ‘ in this way the axis of 
* Trans. Microscopical Society of London, 1861, p. 37. 
