178 METALLURGICAL MICROSCOPE 



Another practice which should be no less condemned when using 

 the Metallurgical Microscope is that of reflecting the luminous pencil 

 at right angles by means of a total reflection prism placed in the path 

 of the pencil of light, instead of employing a silvered reflecting mirror. 

 The former method completely changes the working of an objective by 

 making the pencil of rays pass through a piece of glass many centi- 

 metres thick. The objective is calculated for working in air and not 

 in glass. 



These errors are not very important when the examination is 

 simply by the naked eye, because the eye has an extraordinarily high 

 degree of accommodation. This, however, is not the case in photo- 

 graphy. Frequently the sharpness of image that ought to be possible 

 where objectives are properly used is far from being obtained. 



To sum up : hitherto Microscopes have only been seriously investi- 

 gated for the examination of transparent objects and it would be 

 highly desirable if this study could be resumed and extended with 

 a view to the examination, by reflection, of polished opaque bodies 

 like metals. 



