ILLUMINATION IN MICRO-METALLOGEAPHY 

 By Henry M. Sayers. 



In discussing this subject before this audience it is only necessary 

 to set out certain propositions as assumed and accepted to give form 

 and cohesion to the whole treatment. These assumed propositions 

 are as follows : — 



1. Correct illumination is essential to obtain the best results of 

 which the objectives and oculars used are capable. 



2. The principles of correct illumination are the same for the 

 examination of opaque objects such as those studied in metallography 

 and that of the (partially) transparent objects examined by transmitted 

 light. 



3. The illumination which permits of the utilisation of the maxi- 

 mum potential resolving and defining power of any objective is given 

 by an dmage of the source of light projected on to the object, formed 

 by cones of light with apex angles approximating to the angular 

 aperture of the objective in the medium used. 



4. Micro-metallography implies the use of the highest optical power 

 available — though not necessarily in every case — and the use of photo- 

 graphic records. 



5. Micro-metallography implies the use of some form of " vertical 

 illuminator," and of the objective as a condenser, in all but quite 

 low-power work. 



6. It is desirable to shorten photographic exposures as much as 

 possible, especially at high magnifications, and to avoid "glare"' 

 which reduces contrast and obscures fine detail. 



7. The present effective magnification has an upper limit of about 

 1,000 diams., due to the limitation of the N.A. of objectives to 

 a maximum of about 1.4 by the optical constants of the glasses and 

 immersion media available. Any increase in N.A. and effective 

 magnification will require a corresponding increase in illumination 

 efficiency. 



Proposition 5 sets out two conditions which constitute important 

 differences in the application of correct illumination to opaque and 

 to transparent objects, and give rise to the principal difficulties in 

 attaining good illumination in metallography. The vertical illuminator 

 entails a great loss of light. The use of the objective as the condenser 

 also entails limitations which do not arise in the same degree when 

 these two components of the optical system are independent. 



If a total reflection prism is used as the vertical illuminator it may 

 reflect nearly 100 per cent, of the light falling on it into the objective, 

 but it intercepts all the light from the objective passing towards the 

 image plane which falls on it. In the best case the prism occupies 

 half the objective aperture, only half the objective receives light from 

 it, and half the light going to th^ image is intercepted. Hence only 

 25 per cent, of the illuminating beam can be utilised in the image. If 

 the prism is either larger or smaller the percentage of light getting 

 through to the image is less than 25 per cent. 



166 



