On the Evolution of the Fine Adjustment. By E. M. Nelson. 3C9 
it is necessary to examine carefully all the separate movements and 
various parts, and see if they embody any mechanical contrivances 
that have added their quota to the evolution of the modern Micro- 
scope. Cornelius Varley’s work is to be found in the Transactions 
of the Society of Arts. A great number of the plates, both of 
Microscopes and of a large assortment of machines used for various 
industrial purposes, were drawn by him. Respecting the Microscope 
drawings, with which only we are concerned, they are the best 
drawings that have ever been published on the subject, plans, eleva- 
tions, and sections being given of the more complicated parts. So 
at the outset we see that whatever failings Yarley may have had, 
ignorance of mechanical principles was not one of them. 
The following is an extract from the opening lines of his first 
paper (there is no date to this paper, but Dr. Solly’s supplement to 
it is dated 1831) : — “ Very early in life I was employed in making 
Microscopes and the lenses for them.” . . . . “ On this account, 
and the defective adaptation of most Microscopes for particular 
purposes, I again made one for myself, believing I could fit it much 
better than those hitherto in use, for viewing living objects with the 
very highest powers.” 
From the above we may safely infer that this Microscope, together 
with its lenses, was made by Cornelius Varley(39) not later than 
1831. The instrument was a simple non-achromatic Microscope, 
achromatism not having become completely established in this country 
at that date. It is not my intention to wander outside the scope of 
this paper by describing this instrument, but will merely state in 
passing that it was the first Microscope to have a calotte rotating 
nose-piece and a cylinder diaphragm, both of which contrivances are 
used at the present time. The coarse adjustment is performed by 
sliding the stage up and down the pillar, and fixing it by means of a 
clamping-screw. But it is with the fine adjustment that we are 
concerned at present ; in this Microscope it is of the direct-acting 
screw type, and the milled head, by which it is set in action, is placed 
at the lower end of the pillar. Yarley says, “ There are two faults 
attendant on this screw, both of which will increase, unless counter- 
acted ; one is wearing loose at the shoulder which confines it to the 
bottom, the other is a wearing of the threads of the solid and hollow 
screw : each of these causes a loss of motion, by permitting the screw 
to turn a little without moving the sliding tube. I will therefore 
describe the means of curing these defects : To remove all end-shake 
that would occur from the shoulder wearing, a short spiral or other 
spring is placed on the milled nut to act between it and the seat on 
which the screw-holder rests ; this keeps the screw always down on 
the seat, and thus prevents end-shake, whether the screw is turned one 
way or the other. The other fault, which is occasioned by the screw 
wearing loose in its hole, thereby enabling it to make a portion of a 
turn without pushing or pulling the tube, is sometimes coarsely cured 
