ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



469 



3 mm. thick with 5 mm. radius, 2 mm. thick with 12 mm. radius. The 

 polarizer need not have a field of more than 20"^. For sections of from 

 0' 1-0 01 mm. thickness a fourth lens of 13 mm. diameter and 4*5 mm. 

 focal length is added to the objective, and to obviate the difficulty of 

 mounting very small fragments of crystals for the measurement of the axial 

 angle, this fourth lens, together with the eye-piece and analyser, is made to 

 turn about an axis perpendicular to the axis of the Microscope, and passing 

 through the section, the angle of rotation being measurable. 



In making the measurement the whole body-tube is depressed until the 

 objective is in contact with the section in the immersion liquid ; adjustment 

 to a satisfactory part of the section is then made by the eye-piece tube, and 

 the upper part of the tube is raised until the interference curves are seen ; 

 the angle is then measured by rotation about the axis mentioned above. 

 Since a certain rotation of the upper part of the tube corresponds to the 

 angle of total reflection, this disposition of the instrument renders it pos- 

 sible to measure the index of refraction at the same time. 



On the same principle M. Bertrand has constructed a new refractometer 

 for rock sections.* The rotation of the upper part of the tube is here 

 replaced by a rotation of the hemispherical lens, which is now fixed to tho 

 axis of a small goniometer, carried by a separate pillar mounted on tho 

 Microscope-stand. The objective of the Microscope consists of an achro- 

 matic lens of 30 mm. focal length, and above it is a diaphragm with a 

 slit 1/4-1/2 mm. in breadth, and 3 mm. in length, parallel to the axis of the 

 goniometer. 



The section together with the polarizer is kept in contact with the 

 hemispherical lens by a spring. When the limit of total reflection is 

 reached by a rotation of the goniometer axis, the upper part of the section 

 is bright and the lower part dark, so that the boundary line may be adjusted 

 to the cross wire. The section is illuminated from above by means of a hole 

 in a screen which allows the light to fall only upon the mineral under 

 examination. When the instrument is carefully adjusted, this method will 

 give the refractive index correct to 2 or 3 units in the third decimal place. 



Bertrand's Refractometer. f— This instrument, designed by M. E. 

 Bertrand, may be used for solids or liqaids, and gives the index correct 

 to two places of decimals 

 by a single reading. Fig. 111. 



A B, fig. Ill, is tbe eye- f H c 



piece carrying a lens of f—— ~ rJ_L 



crown glass of 4 cm. focus ; 

 it slides in the tube C D 

 which is conical at the 

 further end, and is pro- 

 vided with a reticule K 

 consisting of a glass disc 

 8 mm. in diameter engraved 

 with 80 divisions, 1/10 

 mm. apart and numbered 



by tens. C D slides in the tube E F F H, the lower face of which is an 

 elliptical section, making an angle of 30° with the axis, and carrying the 

 hemispherical flint-glass lens L of 5 mm. radius fixed in a copper disc. 

 The plane surface of this lens faces outwards, and its centre is in the axis 



* Bull. Soc. Min. de France, viii. (1885) pp. 426-8, and ix (1886) pp. 15-21. 

 t Op. cit., viii. (1885) pp. 375-7. Of. Le Genie Oivil, and Eng. Mech., xliii. (1886) 

 p. 453 (1 fig.). 



