364 SUMMARY OF CURRENT RESEARCHES. 



inserted in the Microscope-tube. This nicol need not be rotatory as 

 it keeps its place unchanged, and needs be once for all orientated within 

 1° to 3°, until the bands attain their maximum sharpness. The Savart's 

 plate is inserted over the Microscope objective 0, either in the objective 

 itself or in a specially rotary ring, and must be set with hard wax in 

 such an orientation that the bands appear in the middle of the field. 

 An achromatic lens of 6-9 cm. focus is used as the Microscope objective. 

 The calcspar parallelopiped K of 3*0 cm. long and 1*3 by 1*3 cm. 

 cross section, with plane-parallel faces, is fastened in a tube which is 

 screwed on to a round brass plate of G mm. in thickness. At the lower 

 end of the tube the diaphragm with the two slits is applied. In the 

 brass plate there is a rather long incision E for the insertion and with- 

 drawal of the smoked-glass wedge. The brass plate is fastened on to a 

 larger plate by means of two clamps ; and when these are tightened up 

 the calcspar and slits can be pushed about until they are in the centre 

 of the field. Under the larger plate two grooved bars P (fig. 75) are 

 attached for receiving a frame with the substance to be examined and 

 brought before one of the openings. On the brass foot M there is a 

 holder T for bearing the divided circle, reading to 5' and carrying a 

 Thompson or a Leppich nicol. The axis of the Microscope can be 

 made perpendicular, within -}°, to the calcspar crystal planes, either by 

 sloping the Microscope-tube or by adjusting the crystal itself. Obser- 

 vations with the wedge are recommended as the most rapid. The 

 wedge is 7*1 cm. long and has a thickness tapering from 0*35 to 

 0*1 cm., and on the side of its metal mount is a millimetre scale. Full 

 instructions are given for gauging the wedge for making observations. 

 The light-source was usually homogeneous ; but sometimes, as in case 

 of crystals, it is required to take measurements along the whole spec- 

 trum ; and then a spectral apparatus similar to "Wulfing's was employed, 

 and a good Welsbach or acetylene light used. 



The author,* however, found that a considerable loss of light resulted 

 from the use of the spectrum apparatus, and he has therefore replaced 

 it by an arrangement which resembles an ocular spectroscope without a 

 second slit. Between the analyser and telescope-tube he places a tube 

 with a small upright prism, whose end-planes are inclined at about 45° 

 to the microscopic axis. A second tube is set perpendicularly to the 

 side of the first and contains a lens (focal length 3 cm.) and a glass 

 scale, the scale being at the focus, so that scale and spectrum are seen 

 together. He found that the brightness was then so great that, even 

 with a small incandescent light, he could measure from A = - 690 to 

 A = 0-430. 



(6) Miscellaneous. 



Comparison of British and Metrical Measures at the same Tem- 

 perature. Computed from the coefficient given in the Report of the 

 Standards Commission, 1871-2, by E. M. Nelson. 



* Op. cit., xxii. (1902) pp. 88-9. 



