LIGHT (OPTICAL) MICROSCOPY 



to have the specimen stay accurately on 

 center as it is rotated. The polarizer, 

 mounted beneath the substage condenser, 

 and the analyzer mounted above the objec- 

 tive are rotatable, and provided with circular 

 scales to read their orientation. 



Beneath the analyzer is an accessory slot, 

 in which one may insert measuring acces- 

 sories, such as a quarterwave plate, a sensi- 

 tive tint plate, a quartz wedge, or a tilting 

 calcite plate compensator. The first two of 

 these devices are used to categorize crystals 

 into one of several classifications. The 

 quartz wedge refines this classification into 

 semiquantitative sub-classifications, while 

 the tilting calcite plate gives actual quan- 

 titative measurements for more refined clas- 

 sification. 



Above the analyzer is an important ele- 

 ment of the Petrographic Microscope, known 

 as the 'Bertrand Lens'. This lens forms an 

 image of the back aperture of the objective 

 in the eyepiece focal plane, so that the ob- 

 server sees the aperture rather than the field 

 of the microscope system. A crystal, located 

 between high N.A. condenser and objective, 

 is traversed by polarized light in many di- 

 rections. In each direction, its properties dif- 

 fer, and it accordingly transforms the state 

 of polarization differently, so that between 

 crossed polarizer and analyzer the crystal ex- 

 hibits a characteristic light pattern which 

 varies with direction. Looking at the aper- 

 ture of the objective, one sees this pattern of 

 light and shade, known as the ''interference 

 figure", and gains a clue as to the nature of 

 the crystal. 



The interference figure of a crystal may 

 be further investigated in a quantitative 

 manner by means of an accessory called the 

 'Universal stage'. This device, located on the 

 stage of the microscope, permits turning and 

 rotating the crystal about several different 

 axes. Crystals have either one or two direc- 

 tions called 'optic axes' in which they behave 

 as non-crystals. By means of the Universal 

 Stage, the positions of these optic axes may 



be determined, giving a very good clue as to 

 the composition of the crystal. 



For a more complete understanding of 

 polarized light microscopy, the reader is 

 referred to the following texts: "Crystallog- 

 raphy and Practical Crystal Measurements", 

 A. E. H. Tutton. "Microscopic Character of 

 Artificial Minerals", A. N. Winchell. "Hand- 

 book of Chemical Microscopy", E. M. Cha- 

 mot and C. W. Mason. "Optical Mineral- 

 ogy", Rogers and Kerr. 



James R. Benford 



ORIGIN AND HISTORY 



Development of Optics and Microscopy 

 from About 1600 to 1723 



During the first years after the invention 

 the words "perspicillum" or "conspicilium" 

 were used both for the telescope and the mi- 

 croscope. What was meant in each case had 

 to be understood from the context. For the 

 microscope also "smicroscope" was used or 

 "engyscope" and other words. Eventually 

 the names familiar to us were accepted about 

 1615. They originated with the Academica 

 dei Lyncei which had been founded in 1003. 

 Allegedly the names were invented either 

 by Giovanni Faber or Giovanni Desmici- 

 anus, who were both members of the Acad- 

 emy and whose names appear in writings 

 from that time. 



Johannes Kepler (1571-1630) is better 

 known for his achievements in astronomy 

 but he also studied intensively optical phe- 

 nomena. In his Paralipomena, printed in 

 1604, he treated the nature of light and color, 

 the position of images, refraction, function 

 of the eye, and other subjects. Color is ex- 

 plained as light partially buried in the ma- 

 terial of the medium which can vary in dens- 

 ity, transparency, opacity and the various 

 degrees of "the light inherent in the ma- 

 terial." Kepler labored hard to determine the 

 law of refraction. He experimented with 

 many transparent media, as air, water, glass, 

 turpentine, vinegar, wine, several oils, and 



454 



