150 



SUMMAKY OF CURBENT RESEARCHES RELATING TO 



until 1690, while Schott wrote in 1657, but this cannot militate against the 

 striking evidence furnished by the three figures. Traber, who lived at 

 Vienna, may well have heard from Schott or otherwise of the mistake that 

 had been made in the drawings, and corrected it accordingly. 



Schieflferdecker's Fine- Adjustment Screw.* — Dr. P. Schiefferdecker 

 describes a micrometer-screw made by Winkel of Gottingen, which is so 

 constructed, that lateral movement is altogether prevented, and the action 

 of the screw is veiy regular and easy. 



Fig. 17 shows a section of the apparatus viewed from behind. The casing 

 which carries the tube is fixed by means of an arm to a hollow trilateral 



Fig. 17. 



Ftg. 18. 



"prism" a. The sides, i.e. the right and left surfaces of the latter, are 

 with the arm made of one piece ; the base, i. e. the hinder surface, is screwed 

 on to the adjacent sides. In a is a second trilateral prism h, the lower end 

 of which is screwed to the foot of the Microscope. This prism is fitted 

 most accurately into the cavity of the former, so that a relatively large 

 friction resistance exists when the two prisms work against each other. 

 The prism h has a cylindrical cavity c, beginning at the top and going 

 down a definite distance. Its axis coincides with that of the prisms. It 

 contains a strong spiral spring, the diameter of which coincides with that of 

 the cavity. To the upper end of c is screwed a hollow steel tube d, the 

 internal diameter of which is equal to that of c. It projects above &, 

 and enters the circular opening of the brass plate /, which lies above a and 

 closes it. The steel tube d is not uniform throughout its extent. After 

 that portion, about 6 mm., which is immediately above h, there follows a 

 part of 11 mm. in length, from which the right and left fourths of its wall 

 have been cut away. On this follows a solid end-piece perforated by the 

 micrometer-screw e. Through the openings in d passes a small brass plate 

 or bridge g, which is fixed at each end to the plate / by a screw. 



If fig. 17 be compared vi^ith fig. 18, the position of this bridge will be 

 understood. The spiral spring presses strongly against the bridge which is 

 firmly united to the plate /; tljis, again, is firmly fixed to a, which carries 

 the tube. The spiral spring therefore exerts its pressure on the uj^per 

 end of a, pushes on this and the tube, and presses the bridge firmly against 

 the upper solid part of d. The micrometer-screw opposes the tension of the 

 spiral spring. It presses on the bridge, not however directly, but by means 



* Zcifs--lir. f. Wiss. Mikr., iii. (1886) pp. 1-5 (2 figs.). 



