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SUMMARY OF CURRENT RESEARCHES RELATING TO 
the instrument is perfectly continuous, and no centering or adjustment 
is required.’ Mr. Jackson also introduced the double pillar; the care- 
fully turned and correctly proportioned pillar was very likely to emanate 
from one who was well known to be such an expert with the lathe and 
other mechanical contrivances. You will notice that Mr. Jackson’s 
improvements were far later in point of time than those of Mr. Lister ; 
for if you will examine Mr. James Smith’s Microscope that was made 
to the order of the Microscopical Society of London and delivered in 
November 1841,* you will see that it has the Lister limb, but neither 
the Jackson ploughed groove nor the Jackson double pillar. 
“ Thus a matter is explained, which for long I have been unable to 
understand, viz. : — why did Messrs. Smith and Beck never supply center- 
ing motions to the substages of their former models? The whole 
thing is clear now ; for at the time those Microscopes were designed 
the necessity for extreme accuracy in centering had not arisen. When, 
however, objectives increased in power and aperture, centering gear was 
provided, in the form of an adapter which held the substage condenser, 
and which fitted into the substage. Other makers, who were not ham- 
pered with the idea of the sufficiency of the Jackson ploughed groove, 
fitted centering motions to the substage itself, and thus dispensed with 
the adapter. For the future, I intend to call all such mounts as the 
one before you this evening, by the name of the ‘ Lister limb,’ for he 
most certainly was its inventor. 
“ The question will be asked, why did Mr. Lister take the trouble to 
invent a new form of stand, when he had the excellent models of Jones 
of Holborn ready to his hand ? The answer to this is that just at the 
close of non-achromatism, the prevailing idea was that every first class 
Microscope should be, in the terminology of those days, both single and 
double, or as we should now say, simple and compound. Microscopes 
were therefore constructed with this idea in view ; and to meet the 
requirements of a single Microscope, the transverse arm was pivoted so 
that the single lens might be made to traverse quickly over a stage, in 
the same way as a c loup ’ on a modern dissecting stand does at the 
present time. In addition to this, the acting portion of the arm could 
be lengthened or made shorter, either by a sliding arrangement, or, as in 
the most highly finished instruments, by rackwork. 
“Now it is obvious that all these movements were conducive to 
instability, especially when the compound body was screwed on to the 
arm above the single lens. Mr. Lister designed his rigid limb with the 
view of correcting these errors in the instruments of his time. 
“ In this he showed great foresight ; for he evidently recognised the 
fact that the introduction of achromatism would so enhance the capa- 
bilities of the compound Microscope as to render its combination with 
the simple Microscope simply impossible, and therefore he determined 
that his instrument should be a compound Microscope, and that only. 
We of to-day may find certain points in his design deserving of criticism, 
such for instance as the steadying rods fixed to the eye-end, which evi- 
dence want of poise, &c. ; but then we must charitably remember that 
the man who makes no mistakes makes nothing, and must give Mr. Lister 
* Microscopical Journal, by Cooper and Busk, for 1842, frontispiece. 
