1893.] MICROSCOPICAL JOURNAL. 169 



or bands of from two to twenty parallel fibres. After a moment's 

 study with a ^ in., of a mount like this, where the coils are drawn 

 out and more or less " uncoiled" into bands, every intelligent ob- 

 server will know the number of fibres in each band in the tangle, 

 which, over the greater portion of the field, cannot but be seen. 

 But take a section with the spirals in situ and undisturbed, or if 

 using a teased-out specimen like this take the first view with the 

 i-5th obj., find at once a still closely coiled cylinder, include in 

 the field onlv that portion which is quite symmetrical and which 

 shows no hint of splitting into bands, or if that be seen, disregard 

 it entirelv, and set out to determine the number of fibres wholly 

 by their slant or pitch in the coil ; and not as a clever guess, but 

 as an absolute fact which nothing could make you doubt. Any 

 child can distinguish a 2-thread screw from a 9-thread one ; but 

 to decide thus between one of these close spirals as formed of 

 5 or 6 threads is next to impossible without special instrumental 

 appliances, and even with them is fiir from as easy as it appears, 

 on account of the curvature of the lines, the constant change of 

 plane, and the lack of absolute mechanical regularity. The simplest 

 available aid in this case would be a cover-glass dropped into the 

 position of an ocular micrometer, in the focus of the eye-lens, 

 marked with a straight scratch with a writing diamond, or with 

 a fine line of India-ink across the centi'e, to be set exactly trans- 

 versely as a straight-edge across the coil. A much better aid 

 to the eye is the common ruled ocular micrometer with its parallel 

 lines, and often with a transverse line or edge at right angles to 

 them ; though an awkward want of conformity in the spacing of 

 the lines can only be avoided bv using a large variety of rulings 

 or of objectives, or bv resorting to inconvenient lengths of draw- 

 tube. By far the best aid of all is the cobweb micrometer, whose 

 pair of parallel threads can be readily set to exactly take in any 

 desired number of the slantinof convolutions. 



Dust at Sea 200 Miles from Coast of Africa. 



From Note-book Q^ of the American Postal Microscopical Chib. 

 By R. H. ward, M. D., 



TKOY, N. Y. 



This interesting specimen of ocean dust might almost be called 

 a '• fossil earth," being so full of broken diatoms and other micro- 

 organisms which can be easily recognized with a i-5th obj. It 

 may well have been whirled up, along with more or less of coarse 

 sand, from some dry spot on the African coast, drifted along in 

 the higher levels of air, separated gradually from the coarser par- 

 ticles by their earlier subsidence, and at last precipitated quietly, 

 during those calm, cool, moist days, in a typical '' infusorial 

 shower," such as has been occasionally observed since the days of 

 Ehrenberg. 



Neither the distance of 200 miles from the coast nor oi 700 



