120 THE REQUIREMENTS OF THE PETROLOGICAL 



taneously with the lower nicol or polariser or independently of it, 

 and there should be special facilities for adjusting it at small angles 

 of divergence from 3 to 6 degrees from the position of crossed nicols.* 

 This is useful in determining the exact position of extinction. 



Greater facilities should be given for the study of the interference 

 figures in the " directions image " in polarised light. It is difhcult to 

 exaggerate the value of the purely qualitative results described by 

 Beck, a"s well as the quantitative methods which involve careful 

 measurements of the " isogyres " or dark bars, f For these purposes 

 immersion objectives with an especially wide angle should be used 

 with highly refracting liquids, and a corresponding wide-angled 

 illumination should be provided. It is absolutely necessary that the 

 student should be in a position to isolate the light from minute 

 crystals surrounded by others of different composition or with differ- 

 ent orientation. Among othr examples may be mentioned the zones 

 and twin lamellae of plagioclase. By far the best means of effecting 

 this is by inserting a diaphragm in the focus of the eye-piece and 

 a Becke lens placed above it.t This should be a recognised accessory 

 with all except the most elementary petrological microscopes. Pro- 

 vision should be made to enable the exact course of the isogyres to 

 be measured. There is no space here to discuss the mei'its of the 

 different devices which have been suggested, including one for which 

 I am responsible. § 



Some arrangements should also be available for the study of the 

 object in linear convergent light, which is advantageous for various 

 purposes. It can be obtained by employing an ordinary convergent 

 system and inserting a narrow slit in a focus conjugate to infinity, with 

 such orientation relatively to the object as may be required. || 



Provision should also be made for the use of monochromatic light 

 when desired. The slit already referred to may be employed for the 

 purpose in conjunction with a prism ; or some form of monochromator, 

 or a colour screen may be substituted, unless coloured flames be 

 preferred. 



I have not attempted to deal with all the numerous accessories 

 which have been employed or suggested in petrological work, but 

 have confined myself mainly to variations of construction necessitated 

 by special methods. 



Reference may be made to the report of the Microscope Committee 

 of the British Science Guild giving a specification of a student's petro- 

 logical microscope.^ 



A serious difficulty is presented by the high cost of petrological 

 microscopes constructed so as to allow of the application of advanced 

 methods of research. This is inevitable so long as the number of 

 instruments inanufactured is too small to justify the employment 

 of systematic standardisation with interchangeable parts. 



"•■ F. E. Wright, Am. Journ. Sci., Vol. 26., ipp. 340-368, 380-386 (iqoS). 

 t M.in. Mag., Vol. XIV, pp. 230-234, 276-281 (1Q07); Min. Pair. Mitt., 

 Vol. XXIV, pp. 1-34 doos). 



t Min. Mag.. Vol. XVHI, pp. 45-51 (1916). 



§ VTin era logical Magazine, Vol. XVIII, pp. 52-57 (1916), 



II Min. Mag., Vol. XVIII, pp. 130-132 (1917). 



51 Journ. Brit. Sci. Guild; November, 1916, pp. 28-31. 



