LIGHT (OPTICAL) MICROSCOPY 



Fig. 1. Microscopic silhouettes of ash sample at increasing temperatures. 



With the aid of the heating microscope the 

 individual phases of the process may be ob- 

 served and recorded photomicrographically. 



The image of the specimen located in the 

 electric furnace is magnified by the micro- 

 scope and projected onto a ground glass 

 screen, where it is observed. The characteris- 

 tic phases are easily photographed. Thus the 

 entire cycle can be observed, the series of 

 photomicrographs giving an impression of 

 the behavior pattern of the material being 

 investigated. By evaluating the volume 

 changes of the specimen a melting curve in 

 relation to temperature is easily plotted. 



One of the principal advantages of this 

 method of investigation is that the specimen 

 at no time is subjected to pressure by a prob- 

 ing rod. Thus, since no load is applied to the 

 specimen, not only the softening process, but 

 also any desired phase of the alteration in 

 shape of the specimen can be investigated 

 and recorded, such as the swelling of such 

 material as ashes as well as the "melting 

 point," the so-called hemisphere point. 



In Figure 1, a series of photomicrographs 

 made with a Leitz instrument showing sil- 

 houettes of the specimen, the individual 

 characteristic alterations in shape necessary 

 for judging the thermal behavior pattern of 

 fuel ashes are clearly seen. The original shape 

 of the specimen is shown in the first photo- 

 micrograph of the series. The next ones 

 show that sintering occurs between 960° and 

 1160° C; the fact that both diameter and 

 height of the specimen are reduced shows 

 that sintering and not softening has taken 

 place. At 1180° C the outlines of the silhou- 

 ette have changed noticeably, indicating that 



the ashes of this stage have passed into the 

 softening phase. The softening temperature, 

 i.e. the temperature at which softening com- 

 mences is thus found to be between 1160° 

 and 1180° C. After swelling slightly at 

 1210° C. the specimen eventually melts. The 

 photomicrograph made at 1270° C. shows the 

 "hemisphere point," the officially accepted 

 melting point. At 1430° C the ashes begin 

 to liquefy and at 1470° C they have flowed 

 uniformly to all sides. (From a brochure 

 published by Ernst Leitz, Wetzlar, Ger- 

 many.) 



REFERENCES 



1. Habell, K. J., AND Cox, Arthur, "Engineer- 

 ing Optics," Pitman and Sons, Ltd., London, 

 1956, pp. 411. 



G. L. Clark 



FIBERS (TEXTILE). See GENERAL MICROSCOPY, 

 p. 343. 



HARDNESS TESTS. See ENGINEERING MI- 

 CROSCOPES, p. 438. 



HEATING MICROSCOPES. See ENGINEERING 

 MICROSCOPES, p. 439. 



INDUSTRIAL RESEARCH, APPLICATION TO. See 

 GENERAL MICROSCOPY, p. 363. 



INTROSCOPE. See ENGINEERING MICRO- 

 SCOPES, p. 438. 



MAGNETOGRAPHY: THE MICROSCOPY OF 

 MAGNETISM* 



Although the phenomena of magnetism 

 have been known for many centuries and 



* Reprinted by permission from the Bell Lab- 

 oratories Record, 35, No. 5, pp. 175-8, 1957. Photo- 

 graphs courtesy of Bell Telephone Laboratories. 



440 



