ISO 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



up a critical image with the edge of the flame in focus and central to 

 the field, and open the diaphragm to its full extent. 3. By means of the 

 Bubstage centring screws move the condenser so that the image of the 

 flame lies just outside the field of a high-power eye-piece. If the 

 stria- are within the grip of the object-glass they will be resolved. This 

 kind of illumination will be of service, for it will enable an observer to 

 obtain high resolution, with a dry condenser, in an instant, without the 

 troublesome manipulations usually necessary. 



(3) Illuminating- and other Apparatus. 



Hutchinson Co-ordinate Micrometer.*— This instrument (fig. 17), 

 made by J. Swift and Son, is a modified form of the ordinary ocular 

 used with penological Microscopes. In place of the cross-wires' usually 

 fitted, is mounted a glass plate, on which two systems of lines are ruled 

 at right angles. In the illustration only the centre of the micrometer is 



Fig. 17. 



shown. These lines are ' 1 mm. apart, every fifth line being slightly 

 thicker than the others, and every tenth line stronger still, while the 

 two which intersect at right angles in the centre, and which correspond 

 with the cross-wires, are given particular prominence. As the image of 

 the object is projected on the network of lines, the ocular will be fourifl 

 useful for recording observations, as an enlarged drawing of any object 

 can be readily sketched on paper similarly ruled, and its dimensions can 

 be ascertained. By its aid the relative proportions of the various con- 

 stituents of a rock can be estimated, and it is useful for recording the 

 position of emergence of optic axes, and for finding the magnitude of 

 the optic axial angle in bi-axial crystals. It may "also be "applied to 

 the determination of this constant by Becke's method (measurement of 

 the position and shape of the black hyberbolic brushes seen in convergent 

 light), being an efficient substitute both for the revolving table used by 

 Becke, and for the double-screw micrometer recommended by F. E. 

 Wright.f 



* James Swift and Son's Catalogue, 1914, p. 20. 

 t Amer. Journ. Sci., iv. (1907) pp. 24-331. 



