THe MICROSCOPE. oon 
ferior to its only rival, the “American Centennial” model in 
perfection of workmanship and finish, but vastly its superior in 
several. convenient appliances of Mr Bulloch’s own inyven- 
tion, has the fine adjustment screw in the proper place. 
The mechanism consists of a lever within the arm, actuated by 
a screw whose milled head is grooved for photography, and 
graduated to degrees, the lever in its turn acting on a box con- 
taining the pinion of the coarse adjustment, all of which, with 
the entire body, is raised by the screw and lowered by the pres- 
sure of a spiral spring above the box. 
Messrs Bausch & Lomb have devised a fine adjustment which 
works well, and is applied to some of their stands, the lever 
being used on others. It consists of two parallel blades of steel 
fastened by one end to the back of the case, by the other to the 
arm which carries the rack and pinion, the whole being so ar- 
ranged that the springs support the entire body, with an upward 
tension. The only points of contact are at the fine adjustment 
screw and at the ends of the springs. 
The same firm apply to some of their instruments a differ- 
ential screw fine adjustment. In this appliance there are two 
screws the threads of one of which are further apart than those 
of the other, yet both work on the same shaft, one unwinding 
as the other winds. These movements result in an advance 
equal to the difference between the threads, so that by making 
that difference very small, the progressive movement may be 
correspondingly decreased. This mechanical principle, when 
applied to the microscope, makes an admirable fine adjustment 
for use with high powers, since the body may be moved through 
a distance quite imperceptible to the naked eye, yet sufficiently 
affect the focus. A well-made lever movement, however, is less 
complex and expensive, and almost equally sensitive. The fine 
adjustment screw on Mr Bulloch’s Congress stand, by a turn 
through one graduation, lifts the body one one-thousandth of 
an inch. 
In their “Acme” stands M@srs J. W. Queen & Co. apply the 
fine adjustment to the back of the arm, with a system of in- 
ternal rollers that has not been fully explained. The description 
of the stand says: “The body is firmly carried upon rollers, and 
is moved by a lever actuated by a screw at the back of the arm.” 
The adjustment is exceedingly pleasant to use and commend- 
able in its results. 
