1068 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION 



by the application of a gentle heat to the slab until the balsam 

 becomes liquefied, when the section can be pushed with a piece of 

 wire on to a suitable slide of glass. Obviously a drop of balsam 

 should be poured upon the latter before the section is transferred. 

 The slide is then warmed until the balsam becomes liquid, when the 

 superfluous quantity is drawn over the upper surface of the section. 

 "When the section is completely covered with the balsam, a thin 

 clean cover-glass is held for a moment over the spirit flame and laid 

 upon the section. Gentle pressure is then applied to the surface to 

 bring it close down to the section and to remove all air-bubbles. 

 The slide is then allowed to become quite hard, when it may be 

 cleansed with turpentine or alcohol and ether. 



Very porous rocks must first be treated with Canada balsam, in 

 order to give them the consistency necessary for the preparation of 

 thin sections. Isolated mineral grains and sands can be mounted 

 by means of Canada balsam dissolved in chloroform. The slide must 

 not be heated, but evaporation allowed to take place. Another 

 method is described by Thoulet ; 1 whilst very soft or decomposed 

 rocks should be mounted according to Wichmann's proposal. 2 



In the application of the microscope to petrological and minera- 

 logical research the employment of polarised light is constantly re- 

 quired, and various means and appliances are needful for its most 

 advantageous application, which are not required by the ordinary 

 microscopist. Considerable pains have been bestowed by both 

 English and Continental makers to fulfil the requirements, and good 

 instruments are now plentiful. 3 



An instrument designed by Mr. Allan Dick has been brought 

 out by Messrs. J. Swift and Son. As this combines all that experi- 

 ence has led petrologists to consider desirable for mineralogical 

 and petrological investigation, a brief account of it is subjoined. It 

 is specially adapted to the study of the optical properties of minerals 

 generally, and particularly to that of the thin plates of minerals seen 

 in ordinary sections of rocks prepared for microscopical examination. 

 The microscope is shown in fig. 803, but since the engraving was 

 made one or two improvements as to matters of detail have been 

 introduced. 4 



The eyepiece tube is slotted at E to receive the micrometer scale 

 (shown detached at F), and to the tube is hinged the analyser B'. 

 which is capable of independent rotation in the usual manner. 

 Upon the eyepiece tube is mounted a toothed wheel, which gears 

 into another toothed wheel mounted on one end of a rod formed of 

 pinion wire. The stage, in the newest forms, is fitted with a scale 

 of rectangular divisions inserted to act as a finder, and with a roller 

 object-clip (patented by the makers) in place of the usual sliding bar. 

 Below the stage, which has neither sliding nor rotatory movements. 



1 Annales de Chimie et de Physique (5), xx. pp. 362-482. 



2 Tschermak's Miner cdogische und Petrogr. Mitt. Bd. v. 1882, p. 83. 



5 Mr. J. Swift, of Tottenham Court Road, Mr. Watson, of Holborn, London, and 

 Messrs. Henry Crouch, Limited, make suitable instruments. Those constructed by 

 Zeiss, of Jena ; Nachet, of Paris ; Voigt and Hochgesang, of Gb'ttingen ; Fuess, of 

 Berlin; and Hartnack, of Potsdam, can also be recommended. 



4 The instrument is protected by letters patent. 



