THE MICRO VIVISECTION METHOD. 135 



SOURCE OF LIGHT AND LENSES. 



A most serviceable light for microscopic work is the loo-watt 

 horseshoe filament, Edison, Mazda, nitrogen lamp with the blue 

 daylight glass. This lamp, when set in a frosted milk-white glass 

 globe, gives a white light very restful to the eye. 



In selecting lenses care should be taken to procure those pos- 

 sessing the maximum focal length together with the greatest 

 numerical aperture. For this reason the Zeiss apochromatic 

 4 mm. dry lens with a N. A. 0.95 and the 3 mm. oil immersion 

 lens with N. A. 1.40 are excellent for microdissection or for the 

 study of living tissue in general. It is to be remembered that 

 the equivalent focus of a lens does not correspond with the work- 

 ing focal distance. 



A good substage condenser with a working focal distance of 

 8-9 mm. can be made by removing the top lens of the Leitz 

 triple-lens, centering condenser. The beam of light issuing 

 from it completely fills the back lens of a 4 mm. objective with 

 N. A. 0.65 and more than half of a 1/12 oil immersion objective 

 with a N. A. 1.30. This gives enough definition for such de- 

 tailed cytological work as the study of the chromomeres in grass- 

 hopper germ-cells and of the types of protoplasmic granules in 

 marine ova. One may obtain very good condensers with a long 

 working focal distance from those used in projection apparatus 

 where a cooling trough is inserted between the condenser and 

 the microscope stage. 



SETTING UP THE INSTRUMENT. 



The instrument is first clamped to the microscope and the 

 moist chamber, with the edges of its walls smeared with vaseline, 

 placed in the mechanical stage. The needle is then adjusted as 

 follows : Place the arm of the needle in the groove of the carrier 

 with the dissecting tip extending into the moist chamber. Push 

 the plate of the carrier (b, in Fig. 2, page 126) over the arm of the 

 needle and tighten the set screw c just enough to allow one to 

 push the needle with a straight and even movement. The needle 

 is now adjusted by hand so as to bring the tip within the field of 

 a low power objective. The set screw is then tightened to clamp 

 the needle in place. Further centering of the needle is made by 



