224 SUMMARY OF CURRENT RESEARCHES RELATING TO 



known to be quite different. The balls must be mounted just high 

 enough to be a little below the level of the top of the Microscope stage. 

 The secondary of the induction coil is stepped up with a half-pint 

 Leyden jar. An ordinary telegraph key was found most convenient for 

 making and breaking the current. The mineral sections are ground as 

 thin as possible, washed free of balsam, and then cemented around the 

 edges to ordinary slides. The slides are reversed when placed on the 

 Microscope stage, and the section should lie just clear of the balls. 



Some Notes on Laurent Polariscope Readings.* — CI. W. Rolfe and 

 C. Field have made two series of rotation measurements of two standard 

 quartz plates on a Laurent polariscope, one set with the light designed 

 to be used by the Laurent polariscope, sodium-chloride light filtered 

 through a section of bichromate crystal ; the other set with sodium- 

 chloride light passed through a Lippich ray filter. The instrument used 

 was a Laurent "large model," made about 1888 by Leon Laurent, of 

 Paris. 



The authors conclude that it is imperative, in stating that the light 

 factor of a saccharimeter is a certain value, that reference should be 

 made (1) to the exact nature of the light used in the rotation read- 

 ings ; (2) the saccharimetric standard of the scale of the quartz-wedge 

 instrument ; (8) the nature of the substance measured ; and (4) 

 obviously, the temperature at which the comparisons are made. 



Quartz-Plate Readings in Saccharimetry.f — G. W. Rolfe gives 

 his reasons for considering that the Landolt-Lippich polariser is not as 

 satisfactory for instruments for general laboratory use as the Laurent, 

 because the former requires that all extraneous light be rigidly excluded, 

 and seems much more sensitive to small variations in intensity of the 

 sodium flame than does the Lanrent. Only under constant conditions of 

 temperature and light intensity, and with a rigid exclusion of all ex- 

 traneous light, can good results be obtained with the Lippich polariscope. 



A. New Spectrometer : its Uses and Advantages.^ — V. H. Mac- 

 kinney has designed a new form of spectrometer, whose main features 

 may be tabulated as follows :— 



(a) Both tubes are auto-collimating telescopes, and both are made 

 to rotate in the horizontal plane. 



(b) The illuminant is central above the prism table, and hence, once 

 set, is constant on the slits whatever position they may occupy round the 

 circle. There is a shade below to prevent stray light from interfering 

 with the observations. 



The advantages gained by these new features may, to start with, be 

 briefly tabulated as follows : — 



{a) The refractive index of a prism can be determined whatever 

 its angles may be, for the new instrument adapts itself for determin- 

 ing it by any of the following methods : — (1) position of normal 

 incidence ; (2) minimum deviation ; (3) critical incidence ; (4) return 

 path (Abbe) method. 



* Technology Quart. Proc. Soc. Arts, Massachusetts, xviii. (Sept. 1905) pp. 

 219-93. t Tom. cit., pp. 294-9. 



% Paper read before Optical Society, London (Feb. 1906) 8 pp., 13 figs. 



