146 SOME NOTES ON THE METALLURGICAL 



comparing the different types of lenses. Very often the effect of 

 the flare can be overcome by using a combination of plate and 

 developer which gives contrast easily; for example, such methods 

 succeed perfectly with the 24 mm. Zeiss apochromat; with the 

 12 mm., however, as stated above, the flare is so great that the 

 author uses for preference a very good achromatic lens of the same 

 aperture. 



(c) Colour Screens. — As mentioned earlier in the paper, the use 

 of colour screens in metallurgical work is simplified very much, as it 

 is only on very rare occasions that a section is obtained which requires 

 light of some definite wave-length in order to get the best results,, 

 consequently the whole attention can be given to using the light 

 most suited to the lens. 



If the objective is apochromatic, light of any colour may be used, 

 but it is generally advisable to use blue light in preference to green 

 or red (especially with the higher powers), as the resolving power is 

 thereby increased. It is always advisable, however, even with the 

 best apochromats, to focus with the same colour light as is used 

 for photographing. The author's general practice in this case- 

 is to focus with a blue screen in position (generally the 

 Wratten tricolour blue), and then remove the screen and expose 

 on a non-colour sensitive plate (all blue screens increase the exposure 

 rather considerably). This method is perfectly satisfactory for the 

 Zeiss apochromats, even at the highest magnifications. 



With achromatic, or semi-apochromatic lenses, one has not the 



same freedom. Owing to the simpler construction of these lenses 



the correction for -spherical aberration is taken to a high degree 



of perfection for light of one colour only (generally yellow green), 



and the best results are only obtainable by using this colour. The 



author has examined such objectives made by most of the leading 



makers in England, and has never met one in which the correction 



for spherical aberration for blue violet light was sufficiently good 



(compared with that for green light) to make it worth while 



taking photographs with such light. Some lenses were certainly 



better than others, and, curiously enough, some of the lenses which 



were very poor with blue light worked quite well with red light. 



The author is of opinion that it would be far better if this fact 



were more widely acknowledged by the makers. To read the 



catalogue descriptions of some of the lenses one would imagine that 



they would perform perfectly without any screen at all. The author 



has known of cases where objectives by well-known English makers 



have been purchased and used in the belief that they would 



perform well under these conditions. After seeing the results the 



purchaser came to the conclusion that the lenses were very poor 



specimens. In one case which occurred recently the author was 



able to convince the purchaser that the type of lens in question 



would give very fine results if used with a suitable colour screen 



instead of in the manner suggested by reading the maker's too 



optimistic description. Probably one of the best screens to use for 



such lenses is one of the tricolour green type. The author uses the 



one made by Wratten, along with the Allochrome plate by the same 



maker. Such a plate as this (sensitive to yellow green) is preferable 



for this purpose to a panchromatic plate, as the red sensitiveness of 



the latter is no advantage — rather the reverse. 



