804 SUMMARY OF CURRENT RESEARCHES RELATING TO 



blackened cardboard was fitted closely over tbo eye-piece of tbe 

 Microscope in sucb a way that tho object was seen through tho 

 radial fissures, the images of tho contraction and relaxation of tho 

 muscular fibrils being thus made perfectly clear and undistorted and 

 easily observed. 



Determining the Thickness of Arterial Walls.* — Dr. H. Stahol 

 uses tho following method for ascertaining tho thickness of arterial 

 walls. 



By means of a micrometer-screw having a thread of 0*5 mm., the 

 milled head of which was divided to permit readings to 0*001 mm., a 

 plato was moved up through the stage aperture, on which a square piece 

 of artery, protected by a cover-glass, was placed. In order to avoid 

 any chance error which might bo caused by tho unequal thickness of 

 tho cover-glass, the centre of tho latter was always placed over the 

 spot to bo measured. The piece of artery and the cover-glass were 

 accurately adjusted to the plate by tho aid of slight pressure. The 

 pieces should not be too large : as a rule, pieces the sides of which 

 were 2 to 3 mm. were used. Vertically over the plato carrying the 

 artery and cover-glass a needle was fastened by a band. The plate 

 was then raised by the micrometer-screw until the needle-point 

 touched the upper surface of the cover-glass. Accurate apposition 

 was obtained by means of a hand-lens. The height was then read off 

 on the micrometer-screw head. The height thus ascertained gave 

 tho thickness of the artery plus that of the cover-glass. The artery 

 was then removed and the thickness of the cover-glass found in a 

 similar manner, and the difference between the two gave the thickness 

 of the arterial wall. 



For the sake of accuracy frequent measurements were taken, and 

 it was found in the result that the thickness of an artery's wall could 

 be ascertained to within 0*01 mm. 



Resolution of Diatoms whose Striae are of unequal fineness.f — 

 Mr. E. M. Nelson writes : — " Every diatom resolver with oblique light 



will have noticed the great difficulty there 

 is (which in some cases amounts to an 

 impossibility) to focus at the same time 

 the longitudinal and transverse striae of 

 some of the Diatomaceae. This will be 

 found especially to be the case in dia- 

 toms whose striae are of unequal fine- 

 ness, such as Navicula cuspidata (36,000 

 and 65,000 per inch). Some observers 

 have maintained that this appearance is 

 the true structure, and that the coarse 

 markings are on the exterior surface of 

 the valve, while the fine are on the in- 

 terior. To those who, like myself, hold 

 that striae are imperfectly resolved perforations, this theory is, of 



* Arch. f. Anat. u. Physiol., 1886 (Anat. Abtheil.) pp. 45-63 (12 figs.), 

 t Eugl. Mech., xliii. (18S6) p. 328. 



