ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 119 



micrometer-wire upon the centre of the image of any line. In 

 examining the spaces seriatim, there v;as some risk of losing count, 

 and as a means of reference, a scale of figures, photographed by 

 Mr. J. Mayall, jun., to the exact length of a centimetre, was pasted 

 at the upper edge of the band, so that the principal graduations of 

 the latter could be identified with a low power. 



" Coming, at last, to the examination of the plate ruled by Prof. 

 Eogers, perhaps its most distinguishing feature is the perfect straight- 

 ness and similarity of the individual lines. The stage micrometers 

 commonly met with are so deficient in this respect, that it is impos- 

 sible to obtain equal distances from different parts of the same two 

 lines of the scale. But with the rulings of Prof. Eogers no such 

 inequality exists. The spider-lines at the eye-piece may be set to any 

 interval of lines on his micrometer, and the scale will rigidly indicate 

 the same distance at any other part of the band, whether above, 

 below, or on either side the position first selected. As to the actual 

 width of the lines themselves, I make it to be '001 mm. almost 

 exactly. After all these precautions for the study of this micrometer, 

 perhaps a list of small, though definite, errata may be looked for ; but 

 I have carefully verified the principal intervals of the band, and a 

 large number, taken at hazard, of the 1000 close spaces, and have 

 detected no discrepancies whatever. The only possible criticism that 

 occurs to me is that the projecting lines at the reading edge are 

 perhaps needlessly long, and that if the ' walking-stick hooks ' could 

 be transferred to the other side of the band, it would be an improve- 

 ment. I believe the ruling to be as accurate as mechanical means 

 can produce ; and though there is no means of deciding whether the 

 spaces are true subdivisions of the French metre, the perfection of 

 the subdivisions themselves is a tolerably sure guarantee that the 

 Professor took every care to verify his unit to begin with." 



Section of "Histology and Microscopy" at the American 

 Association. — At the last meeting of the American Association for the 

 Advancement of Science, a section of " Histology and Microscopy," in 

 place of the previously existing sub-section of Microscopy, was 

 established, to rank on the same footing as the other sections of the 

 Association, and to be represented on the Standing Committee, its 

 Chairman being ex officio a Vice-President. 



Structure of Cotton Fibre.* — Dr. F. H, Bowman has published 

 an elaborate investigation into the structure of cotton fibre, in which 

 he gives a general account of the plant botanically, and deals with the 

 typical structure of a cotton fibre, both in regard to the mechanical 

 arrangement of its ultimate parts, and chemically. A full consideration 

 is given to the variations from the type structure which are found to 

 exist and the extent to which any variation in the ultimate fibre may 

 affect its use in the manufacturing process. 



The book is illustrated with plates of typical and other cotton 



* Bowman, F, H., 'The Structure of the Cotton Fibre in its relation to 

 technical applications,' xvi. and 211 pp., 5 figs, and 12 pis. 8vo, Manchester 

 1881. 



