Science . FlG. 1. Electrode holders supplied with theultra-violet microscope by the maker. Two simpleholders .with screw clamps to hold the wire (orribbon) electrodes. siderable distance below the focal point forthe blue rays. This distance that the object-ive must be lowered is read off on the scaleof the fine adjustment screw of the micro-scope stand. In case of our mm. mono-chromatic objective the focal correctionamounts to forty divisions of the fine adjust- ment screw of the Zeiss Photcimicrographicstand (about mm.). By having the arms of the magnesium elec-trode holders mm.
Science . FlG. 1. Electrode holders supplied with theultra-violet microscope by the maker. Two simpleholders .with screw clamps to hold the wire (orribbon) electrodes. siderable distance below the focal point forthe blue rays. This distance that the object-ive must be lowered is read off on the scaleof the fine adjustment screw of the micro-scope stand. In case of our mm. mono-chromatic objective the focal correctionamounts to forty divisions of the fine adjust- ment screw of the Zeiss Photcimicrographicstand (about mm.). By having the arms of the magnesium elec-trode holders mm. shorter than those forthe cadmium it was found possible to bringthe blue light and the ultra-violet rays to a. Fig. 2. New swing-out electrode holders. Withtwo pairs of holders; the short ones for mag-nesium, the long for cadmium. The holders areopen above so the electrodes can be removed easilyfor adjustment. The lower pair have a stop tobring the electrodes automatically in line. Eitherpair of holders may be thrown into position byturning the handles. About three tenths naturalsize. focus at the same distance beyond the prismsand collector lens, though not in the samespot, as the ultra-violet rays are refractedmuch more than the blue rays in passingthrough the prisms. The illuminating ap-paratus is made to swing laterally as a whole. August 9, 1907] SCIENCE 183 so it is very easy to direct tlae blue or thecadmium rays upon the face of the totallyreflecting prism that throws the light intothe substage condenser. By using two stopsalong the curved way on which it swingsthe illuminating apparatus can be made tostop automatically at the right place to throwthe blue or the ultra-violet light into
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