LIGHT (OPTICAL) MICROSCOPY 



used most frequently by the Laboratories, 

 the rays from the Hght source are directed 

 around the objective, strike an annular mir- 

 ror or condenser, and illuminate the speci- 

 men. This is illustrated in Figure 3. Since a 

 polished magnetic specimen is highly reflect- 

 ing, all incident rays striking the surface are 

 reflected away from the objective, and the 

 field in the microscope appears black. How- 

 ever, the small colloidal magnetite particles, 

 which have already aligned themselves along 

 the domain boundaries, are illuminated by 

 the light impinging on their irregular sur- 



PLANE MIRROR 

 SURFACE 



DARK 

 CENTER-STOP 



-f 



LIGHT 

 SOURCE 



OBJECTIVE 



SURFACE OF 

 ANNULAR MIRROR 



CONDENSER 



COLLOIDAL MAGNETITE ON 

 SURFACE OF SPECIMEN 



Fig. 3. "Darkfield" system of illumination. 

 Light is reflected from the specimen surface 

 through the microscope lens system to the eye. 



faces. Since no light emanates from the back- 

 ground, the reflecting particles appear white. 

 This provides maximum visual or photo- 

 graphic contrast as illustrated in the domain 

 micrograph (Figure 4). 



Over the years, the colloidal magnetite 

 method has been used extensively. In this 

 work, through the efforts of W. O. Baker and 

 F. Winslow of the Laboratories, further re- 

 finement in the preparation of colloidal mag- 

 netite made it possible to see the more deli- 

 cate delineations of the domains, and to 

 observe the changing domain patterns over 

 a much longer period of time. 



Of particular interest was the change in 

 domain patterns caused by various physical 

 influences such as inclusions or small surface 

 defects. It was further observed that irregu- 

 larities in specimen preparation, such as sur- 

 face straining, would cause a type of domain 

 pattern that was not representative of the 

 underlying structure. In addition to present- 

 ing a static picture of domains under the in- 

 fluence of an applied magnetic field, these 

 domains would move with a change in field 



Fig. 4. Magnetic domain patterns obtained on silicon iron by H. J. Williams using the colloidal 

 magnetite method with darkfield illumination. 



442 



