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SOME OBSERVATIONS CONCERNING SUB-STAGE 



ILLUMINATION. 



By S. C. Akehurst, F.R.M.S. 



(Bead January 27th, 1914.) 



Plates 20-22. 



The accepted method, and the one generally used, for sub-stage 

 illumination is that known as the solid cone of light, controlled, 

 within certain limits, by the iris diaphragm. Another form that 

 is, annular light is occasionally used, but is not considered by 

 many microseopists to be of value for critical work. 



Both these forms of illumination are too well known to need 

 detailed explanation. The textbooks, however, have very little 

 to say either for or against the latter method, excepting Cross and 

 Cole, 3rd edition, where a definite statement in favour of annular 

 light is to be found. I cannot do better than quote this : " Stops 

 can be further used for strengthening the contrast in the image 

 with large cones of illumination and objectives having high 

 apertures. This method does not minimise in any way the 

 effective working of the objective, for, with objectives of large 

 aperture, rays may be present which only impart brightness to 

 the field, but do not contribute to making visible the fine detail 

 upon the object. If less than half of the lateral spectra are seen 

 on looking down the tube at the back lens of the object glass with 

 a striated object in focus, then the central portion of the direct 

 beam or central disc has no lateral image corresponding to it in 

 the portions of the spectra that are visible. Under these circum- 

 stances, that central portion of the central disc in no degree 

 contributes in enabling the detail to be seen, but only produces a 

 haze ; by blocking it out the haze is removed and there is a great 



