J. H. G. MONYPENNY 141 



(3) The wave-length of the light used should be that for which 



the objective is corrected. 



(4) The wave-length of the light used should be suitable to the 



colour of the specimen. 



Fortunately, in most metallurgical work the specimens rarely 

 call for the use of any definite colour of light, and hence the neces- 

 sity for complying with condition No. 4 does not, in general, arise. 

 This is a great advantage, as it enables one to adjust the colour of 

 the light to fulfil condition (3). In other branches of microscopic 

 work {e.g., in connection with Biology), it may easily occur that 

 the requirements under conditions (3) and (4) are opposite, and 

 then the photomicrographer has either to use a colour for which the 

 objective is not adequately corrected or which is not best suited to 

 the specimen. 



In metallurgical work the objective acts as condenser, and it is 

 well known that to produce "' critical illumination " the illuminant 

 should be focussed on the section, and should therefore occupy the 



Fig. I. 



Diagram showing- conditions 

 for critical illumination. 



position L shown in Fig. 1, so that the distances x and y are equal. 

 Practically it is found that the illuminant may be to some extent 

 out of focus without producing any bad effect, providing the objective 

 transmits a full solid cone of light. This may be judged by looking 

 at the back lens of the objective after removing the eye-piece, when 

 (the iris on the vertical illuminator being open) the back lens will 

 be full of light and evenly bright if critical illumination has 

 been obtained. The fact that the illuminant may be to some extent 

 out of focus is of great value in allowing one to get rid of the effect 

 of small surface markings on the illuminant itself. 



Placing the illuminant in such a position has obvious disadvan- 

 tages, e.g., it would be inconveniently close to the microscope and 

 the heating effect produced on the latter would be considerable. 

 Again, to illuminate the whole visible field in the microscope, the 

 illuminant would have to have an evenly bright area at least as 

 large as the diaphragm of the ocular in use (say 7 to 8 mm.), 

 obviously, therefore, illuminants of small area {e.g., Nernst or Arc 

 lamps) could not be used in this way. 



