19 



has been added to be placed over the polariser to render the light 

 strongly convergent for producing interference figures in the 

 crystal. 



56. Fuess's petrographic microscope. 



B. 38. 1884. Made by R. Fuess, Berlin. 



This is of the same general type as the last, but of larger and 

 improved form. It is provided with four objectives, No. 3, 5, 7, 

 and 9, which are attached to a revolving nose-piece at the bottom 

 of the microscope tube. This prevents any vertical motion of 

 the tube in its holder, and consequently the coarse adjustment 

 is made by means of screw and rackwork near the base of the 

 stand which raises the stage and its attachments. The fine ad- 

 justment reads to 001 mm. The stage is divided as before, but 

 has a vernier reading to 5'. 



The centring of the axis of rotation is brought about by 

 adjusting the stage instead of the objective. This is done by 

 means of two screws working at right angles, One of these 

 has a graduated milled head, so as to serve as a micrometer 

 whose subdivisions correspond to 002 mm. The polariser as 

 well as the analyser has its circumference graduated, and can 

 be raised or lowered in its containing tube by means of a screw. 

 There are four eyepieces, one of which is provided with a 

 Calderon's stauroscope, as described in Groth's universal appara- 

 tus, instead of the cross lines. The instrument has also Ber- 

 trand's system of lenses for magnifying the interference figures, a 

 Biot- Klein's quartz plate, a quartz wedge, a Bertrand-Lasaulx 

 combination for producing strongly convergent light, and an 

 apparatus for determining the critical points of liquids enclosed 

 in cavities of minute crystals, similar to that for examining 

 heated crystals. 



57. Petrographic microscope. 



E. 115. 1886. Made by A. Nachet, Paris. 



This is the instrument recommended by MM. Fouque and 

 Michel Levy in their Mineralogie Micrographique, Paris, 1879 2 

 where its chief features are described. 



It is supported on two uprights on the usual horseshoe stand,, 

 and rests on a horizontal axis, about which it can be inclined. 

 There are two superstages which move in two directions at right 

 angles over graduated surfaces, so that the position of any object 

 on the slide can be recorded. The principal feature of this 

 microscope is the separation of the objective and eyepiece, which 

 are attached to distinct tubes. The objective carrying tube is 

 connected by an upright with the stage itself. Its position on the 

 upright is controlled by coarse and fine adjustments, and the whole 

 rotates together. Thus the necessity for centring is entirely 

 done away with, the polariser being on a distinct support. The 



