506 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Before stopping-down the field by the diaphragm V just below the 

 Bertrand lens, the image-plane from the objective should, on the same 

 principle, be brought to coincide with the plane of this diaphragm, and 

 the desired mineral section isolated by shutting off light from the 

 adjacent grains. To accomplish this readily, a small lens L (fig. 54), 

 19 mm. focal length, has been introduced in the present Microscope 

 above the Bertrand lens, and, in conjunction with the ocular, serves the 

 purpose of bringing to sharp focus the image-picture in the plane of the 

 Bertrand lens iris-diaphragm, in accord with the principle noted above. 

 In place of this small auxiliary lens, the author has heretofore used a 

 lens of long focal length, and has viewed the Bertrand lens diaphragm 

 directly from the top of the tube. The new arrangement is more con- 

 venient, however, and obviates the necessity of removing the ocular before 

 viewing the interference figure. The lens L swings on an axis, and can 

 be instantly thrown out of the field. A small spring with pointer auto- 

 matically indicates the correct position of the lens when thrown into the 

 field. The Bertrand lens diaphragm ordinarily supplied with Microscopes 

 is of little value in the observation of interference figures by the Lasaulx 

 method without the use of the Bertrand lens, chiefly because of the dis- 

 turbing effects of diffraction from the small apertures required and the 

 distance of the aperture from the eye of the observer. 



Watson and Sons' " Advanced " Penological Microscope.* — 

 The general design of this instrument is that of the Van Heurck model 

 (fig. 57). It is mounted on a similar type of tripod foot, having a spread 

 of 8£ in., and is fitted with rack-and-pinion coarse-adjustment and 

 standard lever fine-adjustment. The stage has a diameter of 4f in. It 

 is fitted with a sliding bar and centring screws. The edge is divided on 

 a silvered surface to degrees reading by a vernier to one minute. A 

 mechanical stage may be supplied to fit on to the surface of the ordinary 

 rotating stage if desired. The body is of large diameter, 2 in., and is 

 fitted with a rackwork draw-tube divided to millimetres ; the lower end 

 has the universal objective thread, so that a low-power Bertrand lens 

 may be inserted for the examination of large crystals, etc. The draw- 

 tube is slotted to receive a Bertrand lens immediately beneath the eye- 

 piece, and the outer body has a suitable corresponding slot to give latitude 

 for adjustment, the actual focusing being done by means of the rackwork 

 draw-tube. At the lower end of the body a large field analyser is fitted, 

 which can be rotated 90°, the rotation reading against a divided scale. 

 Beneath this is a slot to carry a quartz plate or quartz wedge. A quartz 

 plate is included with the instrument. The eye-piece and analyser prism 

 are combined, the latter having a large field and rotating with a reader 

 against a circle divided to degrees. The eye-piece is slotted and has 

 cross webs. The polariser fits into the substage of standard size, and 

 has an extra large prism, the circle by which it is rotated being divided 

 on a silvered edge, and each quarter circle indicated by a spring catch. 



Mounted above the polarizer, but detachable from it at will, is the 

 " lever " high-angle condenser system, either or both of whose lenses 

 can be turned out of the field or brought into position without touching 



* Catalogue, 1910-11, pp. 74-5 (2 figs.). 



