ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



287 



be rotated for purposes of centering, &c. ; (2) a rackwork is applied 

 to the standard by which, the height of the lamp can be adjusted 

 rapidly ; (3) the base is made thinner and the fittings beneath the well 

 altered so that the burner can 



be put 8/4 of an inch lower Fig. 37. 



than formerly ; (4) the oblong 

 frame carrying the lamp-glass 

 (an ordinary 3x1 slip) is pro- 

 vided with two extra grooves, in 

 which may be slid 3x1 slips 

 of tinted or ground glass, or a 

 brass plate (shown in the fig.), 

 to which an adjustable dia- 

 phragm is fitted, can be used 

 in combination with white or 

 tinted light ; (6) the cyliudrica 1 

 part of the chimney is arrange I 

 so that an opal glass reflectoi 

 may be inserted if desired 

 Whilst adding but little to th 

 cost of the lamp as devise I 

 by Mr. Nelson, the new form 

 combines several points of 

 novelty suggested by practical 

 experience. 



Standard Micrometer 

 Scale.* — It will be remem- 

 bered that the American 

 Society of Microseopists ulti- 

 mately abandoned their original 

 micrometric unit of 1/100 mm. 

 and adopted 1/1000 mm. or 1 [x. 

 The United States Bureau of 

 Weights and Measures under- 

 took to prepare and authenti- 

 cate a standard scale, and in 

 August 1882, such a scale, 



ruled on a plafcin-iridiumbar, and verified with great care by Professor 

 C. S. Pierce, was placed at the disposal of the National Committee on 

 Micrometry representing the various Microscopical Societies. A 

 sub-committee for testing this micrometer was appointed, on whose 

 behalf Professor W. A. Eogers subjected the plate to a prolonged and 

 elaborate study which was not completed until August 1883. 



This scale is divided into ten millimetres, each division being 

 marked by three lines distant from one another ten microns, and the 

 measurement is to be made from the mean position of one triplet of 

 lines to that of another. The first millimetre is again divided in the 

 same manner into tenths of millimetres. The first tenth of a milli- 



* Proc. Amer. Soc. Micr., 6th Annual Meet., 1883, pp. 178-200. 



