ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 435 



It is with the use of convergent light for interference figures that 

 the accuracy and simplicity of this instrument become apparent. No 

 centering being required, it is evident that an interference figure once 

 seen will remain in the centre of the field during the entire rotation. 

 If it passes out of view it is on account of the nature of the figure. 

 Even in the case of the smallest crystals, no doubt is ever left in the 

 mind of the observer whether the figure may not have disappeared 

 owing to imperfect centering. 



I have placed on the table two typical sections. The one contains 

 large and well-defined crystals of augite, olivine, and felspar, from the 

 Lion Haunch, near Edinburgh. It will be seen that by pushing any 

 crystal towards the centre of the field till the angle to be measured 

 touches the intersection of the cross wire, a reading of the angle is 

 obtained. Pushing the crystal into the centre of the field, and examin- 

 ing it by convergent light under a high power, it is easy to ascertain the 

 direction of the line joining its optic axes if they can be seen in the 

 section. This is noted, and one of the cross wires brought parallel to 

 it. A reading of the circle is then made, and the rotation continued 

 through 45°, The high power is then replaced by a lower power, and 

 the strongly converging upper lens of the condenser is pushed out so 

 that the mineral may be examined in less strongly convergent light for 

 the purpose of ascertaining in what direction compensation is obtained 

 when the mica or quartz wedge is thrust through the eye-piece parallel 

 or at right angles to the optic axial plane, inclined 45° to the 

 planes of the crossed nicols. By thus studying the emergence of a 

 bisectrix it is seen whether it is positive or negative. The other section 

 consists of a Scotch hornblende-schist. The greater part of the section 

 consists of water-clear granules of quartz and felspar, containing 

 amongst the mosaic a number of well-defined crystals of rutile, and an 

 immense number of less well-defined crystals of some mineral showing 

 very dark borders, due to the fact that its refractive index is much 

 higher or much lower than that of the mosaic. All the grains, except 

 the hornblende and some parts of the mosaic, are under the 1/1000 of 

 an inch, piled upon one another, for the section is a rather thick one 

 except at the edges. In this section are two small grains, one of which 

 shows the emergence of one optic axis of a felspar, whilst the other 

 shows the cross of quartz cut nearly perpendicular to its principal axis. 

 Close to it lies one of the still smaller grains of the more or less highly 

 refractive crystal. It lies flat, gives straight extinction, and shows the 

 nearly perpendicular emergence of an optic axis. I think the mineral 

 is epidote, but draw attention to it merely to show the ease with which 

 interference figures can be studied. To a petrologist accustomed to a 

 rotating stage and fixed cross wires, a familiar section looks strange 

 when first looked at on a fixed stage with movable cross wires, but after 

 a few hours' work with the instrument, the feeling of strangeness passes, 

 and that of the solid advantage of a perfect centering alone remains. 



There is one fact which I should allude to in connection with the 

 small interference figures seen on looking down the tube of the Micro- 

 scope. It is, that the spot of light at the back of the objective in which 

 the figures are seen rotates slightly when the wheels are turned. This 

 is due to its being seen by the extraordinary ray. It may be regarded 

 as a blemish, but is of no practical importance. 



Beneath the stage is a universal fitting, whereby any substage 



