An Improved Vertical Illuminator. By E. M. Nelson. 283 



over the narrow part of the nose-piece, which is 0'935 in. in 

 diameter. 



A spring click (not shown) is provided to indicate when the 

 centre of a hole is in the centre of the aperture in the nose-piece. 

 The square aperture is for the purpose of rendering the illumina- ' 

 tion oblique. With a vertical illuminator minute graduations in 

 the size of the aperture produce little, if any, effect in the image, 

 therefore an iris diaphragm is unnecessary, but the examination 

 of objects under oblique illumination is often of great importance ; 

 with the ordinary kind of vertical illuminator no provision is made 

 for doing this. In this instrument it is only necessary to rotate 

 the tube so that one edge of the square aperture should come over 

 the hole in the nose-piece. 



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