774 RECORD OF CURRENT RESEARCHES RELATING TO 



The second part explains tlie results obtained by tbe application 

 of the Microscope to the study of minerals and rocks, and par- 

 ticularly the light thereby thrown on the actual constitution of the 

 latter, and on the complex structure of a great number of crystals 

 supposed to be simple, their mode of formation and the changes 

 in the temperature, chemical composition, and stability of the 

 media during the process. The Microscope is thus able to give an 

 account of the conditions which prevailed when the minerals were 

 being formed, as well as doubling the field to which geology can 

 extend its conquests. 



The third part gives a summary account of the methods of 

 examination by polarized light, and the modern improvements which 

 have been made in the examination of minerals by its means. 



Adams' Measuring Polariscope. — This consists of three principal 

 parts. The lower section consists of a mirror, a lens, a Nicol's 

 prism, and two other lenses. The upper section consists of lenses 

 and Nicol's prism arranged in the reverse order. Each lens and 

 Nicol's prism is supported separately by screws, and its position can 

 be altered independently of the others. These two parts form a 

 complete polariscope. 



Besides these there is a middle piece, consisting of two lenses 

 (nearly hemispheres) forming a box to enclose the crystal immersed 

 in oil, their curved surfaces being concentric. The whole middle 

 piece is supported on the tubes of the upper and lower portions, and 

 may be turned about the optical axis of the instrument. The vertical 

 graduated circle carrying the central lens and crystal may be turned 

 through an angle about its horizontal axis. By means of an arc 

 fastened perpendicularly on the graduated circle, with its centre at 

 the centre of curvature of the central lenses, the crystal may be turned 

 about another horizontal axis at right angles to the former, so that 

 the crystals and the central lenses can be turned about each by three 

 axes which are mutually at right angles. By means of a system of 

 toothed wheels in gear with the rims of the central lenses, the central 

 and crystal lenses may be turned separately about the optical axis of 

 the instrument, so as to bring the planes of the optic axes of a 

 biaxial crystal parallel to the plane of the vertical graduated circle. 



Homogeneous Immersion. — From conversations which we had 

 with microscopists at the time of Professor Abbe's recent visit, it 

 appears that the difference between the modern " Homogeneous 

 Immersion " and the " Oil-Immersion " of Amici and Hartnack "has 

 not been appreciated. 



One of the leading points of Professor Abbe's theory of 1874 was 

 his explanation of the important bearing which the diffraction pencils 

 have on the formation of the microscopic image so that the resolving 

 power of an object-glass is dependent upon the difiraction pencils that 

 are taken up by it. 



This fact was not previously known, and in the absence of that 

 knowledge it is not surprising that those who suggested the use of oil 

 instead of water abandoned it in practice, not thinking it worth while 



