THE OPTICAL EQUIPMENT 61 



the eye to appreciate. A keen observer will see in a 

 microscopic image all the structure that the best objectives 

 can show with a magnification of 750 to 800 diameters, 

 although it may facilitate observation to amplify the image 

 beyond this. 



The methods of illumination so far described are applicable 

 to transparent objects only ; for opaque objects totally different 

 arrangements must be utilised. Opaque illuminators, of what- 

 ever description, are placed above the microscope stage ; in 

 the case of low powers the position usually is between the 

 object and the objective, and for high powers as near as possible 

 to the posterior combination of the objective. The specimen 

 may be illuminated either by obliquely reflected or normally 

 reflected light. 



In general the principle of illumination of opaque objects is 

 to project on to the surface of the object a reduced image of the 

 illuminant ; as nearly as possible securing the same conditions 

 as in the critical illumination of transparent objects, except 

 that the image formed is not the result of light obstruction, 

 but is entirely due to the differences in absorption or reflection 

 by the structure under observation. The appearance obtained, 

 apart from the structure of the object itself, will vary according 

 to the angle of incidence of the light. If low powers are used, 

 the simplest method is to project on to the object, by means of 

 the ordinary bull's-eye, a beam of light of greater or less obliquity 

 to the optical axis. Assuming that the surface of the object 

 has some irregularities of structure and is not a highly polished 

 metallic surface, then the appearance will be one of alternate 

 areas of light and shade. If, however, the beam is projected 

 vertically and in alignment with the optical axis of the micro- 

 scope, the appearance will be altered and in general a reversal 

 of the light and shade will take place. The result of the 

 alternative methods of illumination at least, with metallic 

 surfaces is practically positive and negative. It is almost 

 entirely due to the demand in micro-metallurgy for improved 

 means of illuminating opaque objects that the existing methods 

 and appliances were originated, and therefore the illustrations 

 are mainly those of metal surfaces. 



Fig. 16 a and b, which are photographs of polished metallic 

 surfaces illuminated by oblique and normally reflected light, 



