ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 287 



of the black line from the thin end of the wedge is proportional to the 

 thickness of the quartz on the stage. 



By substituting for the quartz wedge on the stage mineral sections 

 of uniform thickness cut at the same angle to the optic axis, the distance 

 of the black line from the thin end of the analysing wedge in each case 

 gives a means of estimating the strength of the double-refraction. 



As applied to the examination of minerals contained in rock sections, 

 the method is complicated by the fact that they vary in thickness and 

 also in the angle to an optic axis at which they are sliced. The fact 

 that sections prepared by a skilful lapidary do not differ greatly in 

 thickness, helps to obviate the first difficulty ; and the second is partly 

 overcome by choosing for examination the most brilliantly coloured 

 crystals, which are presumedly those cut approximately parallel to an 

 optic axis. At any rate, the method enables one to separate at a glance 

 such strongly refracting minerals as rutite, dolomite, calcite, sphene, 

 anatose, and zircon. So powerful is the double-refraction of rutite, 

 calcite, and sphene that two wedges are sometimes necessary in order to 

 bring the dark line within the range of vision. 



So also the minerals of very feeble double-refraction are easily 

 separated. In these cases sometimes the black line is on the very edge 

 of the quartz wedge, or is just beyond the range of vision. In the latter 

 case a 1/4 undulation plate is inserted above the object-glass, which has 

 the effect of shifting the spectra up the wedge. 



In ordinary rock sections quartz rarely exhibits more than one 

 chromatic band between the dark line and the edge of the wedge ; 

 whilst such minerals as muscovite, olivine, and actinolite commonly 

 present three and sometimes as many as five such bands. A feeble 

 double-refracting mineral will never exhibit the phenomena presented 

 by one of strong double-refraction, but the latter when cut approximately 

 at right angles to an optic axis will resemble a mineral of feeble double- 

 refraction cut approximately parallel to an optic axis. In this case, 

 however, the mineral will exhibit characteristic appearances when ex- 

 amined in convergent light. 



In cases where a mineral is so minute as to be less in diameter than 

 the width of one of the chromatic bands exhibited by it, the number of 

 bands which come in between the dark line and the thin edge of the 

 wedge can still be counted if, confining his attention to one colour, the 

 observer counts the number of times before extinction that the mineral 

 assumes that colour as the wedge is moved across it. 



As an illustration of the close approach to accuracy obtained by the 

 use of the method the author mentions the case of sphene, which the 

 determinations of refractive indices made by M. Levy and Lacroix show 

 to have a position, as regards intensity of double refraction, betM^een 

 zircon and calcite, a position assigned to it by the author on the evidence 

 afforded by the rough and ready use of the quartz wedge. 



" Method of using with ease Objectives of shortest working dis- 

 tance in the clinical study of Bacteria." * — Dr. A. C. Mercer writes 

 as follows : — 



" The working distance of homogeneous-immersion objectives of 

 short focus and great numerical aperture is little. In the clinical study 

 of bacteria, sputa and other more or less fluid material are generally 



* The Microscope, ix. (1889) p. 46 



