GEOPHYSICAL LABORATORY. 95 



dence over all others in crystallizing out of the magma; magnetite will be 

 the first to crystallize only when it is present in excess of the quantity which 

 can be carried in that particular solution at the temperature and pressure 

 which chance to prevail there. 



In the development of laboratory methods in the service of petrology, one 

 in particular has opened a new field of considerable significance. The ability 

 to detect and locate extremely small energy changes in a cooling or heating 

 system makes it possible to establish inversion temperatures with certainty. 

 These inversions, or changes of crystal form in the solid state, are peculiarly 

 independent of neighboring minerals, or of volatile ingredients, and there- 

 fore often serve to determine the temperature region in which a particular 

 natural mineral must have formed. Such inversions appear to be consider- 

 ably more numerous than has hitherto been supposed, and when established 

 in sufficient number form a kind of geologic thermometer which can be easily 

 and generally applied in the field. 



(6) A new petrographic microscope. Fred. Eugene Wright. Am. Journ. Sci. (4), 28, 

 407. 1910. 



Experience has shown that in microscopic work with artificial silicate 

 specimens special methods and apparatus are necessary, and to meet these 

 requirements a special microscope has been constructed. Its most important 

 features are : ( i ) Both nicols revolve simultaneously, the connection between 

 the two being a rigid bar, thus eliminating the errors due to lost motion in 

 the gear wheels ordinarily employed for this purpose. (2) The upper nicol 

 remains in the tube and the nicols are crossed by inserting the substage 

 polarizer. This arrangement eliminates the annoying change of focus, and 

 therefore of field, ordinarily experienced on insertion of the upper nicol. 

 (3) The sensitive plate is introduced just below the condenser and fits in a 

 carrying device which can be rotated about the axis of the optical system. 

 This disposition has been found convenient in determining the relative ellip- 

 soidal axes in a crystal section, since the sensitive plate can be rotated more 

 quickly than either the microscopic stage or two nicols together. (4) A 

 mechanical stage of novel design and simple but effective mechanical con- 

 struction. The stage is practically dust proof and has a free upper plate 

 with a movement of 24 mm. in any direction. (5) The Bertrand lens is 

 fitted with a sliding device by means of which the magnification of interfer- 

 ence figures can be varied from 6.5 to 15.2 diameters. For the purpose of 

 bringing an interference figure to sharp focus in the plane of the iris dia- 

 phragm immediately below the Bertrand lens, a small auxiliary lens is used 

 in conjunction with the ocular. This lens is attached on a swinging arm and 

 can be thrown in or out of the field at will without disturbing the eyepiece. 



(6) A second iris diaphragm occurs just below the eyepiece and serves in 

 place of the cap diaphragms now used in observation of interference figures 

 after the Lasaulx method without the Bertrand lens. (7) A large Abbe 

 condenser is used, together with an Ahrens prism of 15 mm. edge, or with 

 a large Glan-Thompson prism, in place of the usual nicol and condenser sys- 

 tem with removable upper lens. 



(7) A new ocular for use with the petrographic microscope. Fred. Eugene Wright. 



Am. Journ. Sci. (4), 29, 415. 1910. 



This ocular consists of a Ramsden eyepiece and three mounted edges or 

 plates, a, h, c, which are inserted in the focal plane of the eyepiece, and by 

 means of which the following three fundamental optic properties of minerals 



