96 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
true had their premises been correct, hut the fallacy of the criticism lay 
in the fact that this fine-adjustment is not the same as Zentmayer’s. 
The reason why Zentmayer’s fine-adjustment broke down was because 
it had no sprung grooves; the slides worked in solid Y-shaped grooves, 
so that in more or less time the effect of wear made itself apparent, the 
fitting became loose, and as there was no means of tightening it up again, 
the Microscope in the end became only fit for the proverbial dust-bin. 
The essential point of a Microscope is the springing of the dovetail 
grooves, and, so far as I am aware, it is to Messrs. Powell and Lealand 
that ‘ microscopy ’ is indebted for this valuable invention or adaptation. 
Whether springing of dovetail grooves was previously used in instru- 
ments other than the Microscope I am unable to say, but my impression 
is that Messrs. Powell and Lealand were the first to use it in the Micro- 
scope. Now, in Watson’s Microscope we have two sprung slides, one 
for the coarse-adjustment, and one for the fine. The moment either 
movement exhibits the slightest sign of wear the slack can be imme- 
diately taken up by tightening the screws. There is no reason, there- 
fore, why in years to come this instrument should not work as well as 
it does to-day. There is one point, however, which must be mentioned, 
and that is the weight of the body and of the coarse-adjustment slide is 
thrown on the fine-adjustment lever. It differs, therefore, from Powell’s, 
inasmuch as the fine-adjustment in this instrument moves the whole 
body, whereas in Powell’s it only moves the nose-piece. Strictly speak- 
ing, in this instrument there is no nose-piece at all. In general, a 
Microscope which has much weight on its fine-adjustment is to be 
regarded with suspicion. All who have had much to do with the Micro- 
scope know painfully well how soon the fine-adjustments of the Conti- 
nental Microscopes, which have a considerable weight of brasswork 
thrown on a delicate screw, become useless. Here the Campbell dif- 
ferential screw with its strong threads has come to the rescue. In 
Watson’s instrument w T e have a somewhat similar compensation : the 
arms of the lever being 1 : 4 J, the weight which ultimately falls on the 
fine-adjustment screw is reduced in that proportion. It must be remem- 
bered, too, that we are not now dealing with such large or heavy tubes 
as in the Powell instrument, but with far smaller and lighter tubing. 
The actual weight on the screw is, I am told, a trifle under a quarter 
of a pound,* which is, of course, not excessive. This instrument may be 
said to be identical with what may appropriately be called Swift’s No. 2, 
with this difference : in Swift’s the lever is parallel to the body, and in 
this it is at right angles to it. In Swift’s side-lever No. 1 the instru- 
ment had a nose-piece, which only was moved as in the Powell ; in his 
No. 2, however, both the body and the coarse-adjustment slide were 
moved ; but in his No. 3 or present form only the body is moved. A 
lever at right angles to the body has two advantages over a side-lever, 
the first being that the screw-head is as conveniently placed for use with 
one hand as with the other ; and the second is, that for photomicro- 
graphy, the gearing to the focusing rod is more simple and direct. 
There is one very ingenious and novel adaptation in this instrument 
which I would like to bring to your notice ; the fine-adjustment screw is 
* When the tubes and coarse-adjustment pinion-heads are made of aluminium 
this will be further reduced. 
