ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 659 



To transfer a fixed quantity of blood to the glass cylinder Prof. 

 Flejscbl uses what he calls an " automatic blood-pipette," made by dividing 

 a fine thermometer tube into lengths of equal capacity by sliding a short 

 column of quicksilver from one part to another of the tube and marking 

 the glass at the ends of the column (which is not less than 1 cm. in length) 

 with a diamond. The tube is then cut through at these points, and each 

 length is ground to a conical termination at each end and provided with 

 a short holder of silver wire. If the end of one of these pipettes is 

 immersed in a drop of blood it becomes filled by capillary attraction, and 

 a unit volume of the liquid may thus be transferred to the glass cylinder. 



Measurement by Total Reflection of the Refractive Indices of 

 Microscopic Minerals.* — M. J. Thoulet describes a contrivance for measur- 

 ing the indices of minerals under the Microscope by Kohlrausch's method 

 of total reflection. The only microscopic methods which have been em- 

 ployed with advantage are those of the Due de Chaulnes and of Mallard, 

 but in both of these it is necessary to have a section of the mineral and to 

 determine the thickness of the section with accuracy ; with Kohlrausch's 

 refractometer it is only necessary to have a plane surface of the mineral 

 immersed in a liquid of greater refractive index, so that a natural crystal 

 face may conveniently be employed. In this apparatus, as is well known, 

 the liquid of high refractive index is contained in a cylindrical vessel 

 surrounded by oiled paper, which serves to illuminate the interior with 

 diffused light except at the point occupied by the observing telescope, and 

 the mineral is supported on a rotating axis, which coincides with the axis 

 of the cylinder. When the normal to the crystal surface makes with the 

 axis of the telescope an angle equal to that of total reflection, one-half of 

 the field of view is illuminated by totally reflected rays. The field is 

 consequently divided into two equal parts of very unequal intensity. If 

 the angle between the two positions at which this occurs is 2 i, then i is 

 the angle of total reflection, and the index of the mineral is [x sin i, 

 where /x is the known index of the liquid. 



M. Thoulet's contrivance is merely the total refractometer of Kohlrausch 

 applied in a simple form to the stage of Bertrand's Microscope."]" A plate 

 of blackened brass fixed to the stage by the screws d d carries the graduated 

 semicircle s (of which R is the axis) moved independently by the milled 

 head A, and carrying the vernier t with it when moved by B. This axis 

 carries not only the object o, but also the small cylindrical tube M, into 

 the cork a of which it fits closely. This tube contains the bisulphide of 

 carbon or other liquid which surrounds the object, and being completely 

 closed prevents evaporation. M is surrounded by a second cylindrical 

 tube N, open above, but closed below by the cork P. This tube is fixed 

 to the holder D by a point which enters P ; and D, being attached to the 

 stand of the Microscope by the spring clip C, may be adjusted by hand 

 to any desired position. The tube N is covered with oiled paper except 

 along a narrow band parallel to its axis, which is brought opposite to 

 the objective E by turning the milled head G. 



The tube M having been filled with carbon disulphide, and the object 

 fixed at O with its face parallel to the axis (gum arabic may be used for 

 this purpose, being insoluble in the liquid), the whole apparatus is rapidly 

 centered and adjusted by the stage movements and those at G and C; 

 the angle of total reflection is then determined in monochromatic light 

 by the goniometer, which is divided to tenths of a degree, and which by 



* Bull. Soc. Mineral. France, 1883, pp. 184-91 (1 fig.), 

 t See this Journal, 1883, p. 413. 



