The two knobs with drum are mounted on an axle _a_, which in its central portion is enlarged and provided with a worm gear. The latter in its turn drives the gear wheel, _d_. A spiral spring pressing against one of the journals into which the axle is mounted prevents any lost motion in the worm gear. Mounted on the same shaft with the gear-wheel _d_, is a heartshaped cam _f_. On this cam rests the steel roller _g_, fitted on the support _k_, which in its turn carries the microscope tube. By its weight and a spiral spring the latter presses on the cam _f_, and effects a direct vertical movement. [Illustration] The periphery of the heart shaped cam _f_, is exactly symmetrical and mathematically correct. The distance traversed by its curved sides from the lowest to the highest point and vice versa is 3 mm. There are cut 60 teeth in the entire periphery of the gear-wheel _d_; the number corresponding to one side of the heart-shaped cam or 3 mm elevation, are 30 teeth, therefore 1 tooth means a movement of 3/30 mm or 0,1 mm. This in turn requires one complete revolution of the axle _a_, the drum of which is graduated into 100 parts. The value of each division on the drum is therefore 0,1/100=0,001 mm. This micrometer fine adjustment is exceedingly accurate and reliable, and besides it has the following advantages; By virtue of its unique construction the movement is continuous, there is absolutely no limit to the motion of the micrometer screw, which may be turned forward or backward as the work requires. This movement extends in exact ratio to the revolution of the cam for a distance of 3 mm. As another advantage we may mention, that it is almost impossible to break the coverglass of the specimen with this fine adjustment. Should the objective come in contact with the coverglass and assuming the knob is turned still further then the tube, which is of aluminium and very light would simply rest on the specimen, without breaking the coverglass, as the latter will easily stand the small pressure of the tube and the fine spiral spring. [Illustration] No. $