ZOOLOGY AND BOTANY, MIUKOSCOPY, ETC. 211 



power with dark-ground illuiniiiation, the condenser must have not less 

 than tliree times the N.A. of the objective. As condensers are hmited 

 to, say, 1*40 N.A. this means tliat it is impossible to obtain their 

 fullest resolving power with dark-ground illumination with any ol)- 

 jectives over 0*47 N.A. 2. If a dark-ground illumination has an 

 aperture less than that of the ol)jective, then the limit of resolving 

 power of the combination is measured by one quarter of tlie sum of the 

 numerical apertures of illumination and ol)jective. 8. With dark-ground 

 illumination it is important that the wheel-diaphragm should be only 

 just large enough to secure a dark background, otlierwise there may 

 be certain ranges of structure which cannot be resolved, although both 

 finer and coarser ones are visible. 



Oblique Illumination in Petrographic Microscope Work.* — F. E. 

 Wright summarizes the methods of obtaining oblique illumination in 

 petrographic work, as follows : — 1. By swinging the substage mirror to 

 one side and allowing the light to enter the section under a large angle 

 (condenser and polarizer having l)een previously removed). 2. By a 

 sliding stop in the lower focal-plane of the condenser. 3. By placing 

 the index finger below the condenser and observing the edge of the 

 shadow which it casts. 4. By means of an opaque strip immediately 

 al>ove the upper lens of the condenser. 5. By use of a sliding stop 

 in the Microscope draw-tube. (>. By a shding stop in the eye-circle of 

 the ocular. 



The author describes in detail the above methods, and discusses 

 their relative merits. He concludes that the methods are not all equally 

 good, either from a theoretical or from a practical standpoint. Theo- 

 retically the first three methods are superior to the others ; methods 

 2 and 3 are also simplest to apply in actual work, and, therefore, are to 

 be adopted in preference to the others. 



In discussing the application of oblique illumination to petrography, 

 the author describes the following simple field method for distinguishing 

 calcite and dolomite. The ordinary refractive index, W, of calcite is 

 1*658; that of dolomite l*t)82. Grains of calcite immersed in 

 ~a-monobrom-naphthalene (AV = 1 • 658) show distinct colour fringes ; 

 such colour fringes do not appear along the margins of dolomite grains. 

 To apply the method in the field, two object slides and a small bottle of 

 a-monobrom-naphthalene are necessary. The material to Ije tested is 

 finely powdered with the hammer, and a small portion immersed in a 

 drop of the a-monobrom-naphthalene on the object-slide. The drop 

 and the immersed powder are then covered with the second object-sUde 

 and the whole tilted and examined through a pocket lens pointed to- 

 wards the sky. Oblique illumination is procured by placing the finger 

 in front of the object-slides (between the glass and the sky). The edge 

 of the finger then appears out of focus and indistinct ; grains of calcite 

 within this semi-dark zone of indistinct focus show the characteristic 

 blue and orange marginal colours, while grains of dolomite appear simply 

 white and dark. Here and there a suo^s-estion of colour is to be ob- 



'&{ 



* Amer. Journ. Sci., xxxv. (1913) pp. 63-82. 



